Guidelines for Ramp and Interchange Spacing (2011)

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92 Appendix A Scenario Based Case Studies

This appendix presents five scenario-based case studies that demonstrate how to apply the various ramp and interchange spacing principles within the evaluation framework presented in Chapter 5. The case studies generally follow the conceptual design to refined alternative steps outlined in Chapter 5. Each case study consists of different geometric, operational, safety, and signing characteristics to provide the user with a range of contextual design environments from which to apply the principles presented in these Guidelines. These case studies include site descriptions, photos, and step-by-step discussions on how these Guidelines could be applied. Users should apply professional judgment and adhere to local practice when considering and recommending interchange and ramp spacing values for their own projects. The case studies include the following project scenarios: • Case Study 1: New interchange on a divided highway being upgraded to a full freeway (rural); • Case Study 2: New interchange on a freeway (suburban); • Case Study 3: New interchange on a freeway in a metropolitan area with a one-mile spaced arterial grid (suburban); • Case Study 4: Modernizing an interchange on a vintage freeway (suburban); and, • Case Study 5: New interchange near a system interchange on a high- volume freeway with many ramps (urban). Appendix A 93

Case Study 1 Case Study 1 walks users through the process of assessing ramp and interchange spacing. In the case study, ramps and interchanges are spaced far apart and traffic volumes are low. Users are introduced to concepts which indicate that a new interchange can easily be accommodated on the facility. BACKGROUND General A state transportation agency is upgrading an existing four-lane divided highway (US 32) to a full freeway and removing at-grade intersections. The roadway is in a rural part of the state and far from any large metropolitan areas. Austin Road currently intersects the divided highway at a two-way stop-controlled intersection. The state wants to maintain access to Austin Road after the freeway upgrade is complete by constructing a diamond interchange. A diamond form will have lower costs than other interchange forms considered for this location, and the diamond form will be consistent with the other interchanges in the corridor. Adjacent Interchanges To the east, the nearest interchange is at SR 56, 12,200 ft away from Austin Road (measured between the centerlines of each crossroad). To the west, the nearest interchange is at SR 248, 8,300 ft away. Both interchanges are diamond forms, and there are no at-grade intersections on US 32 between Austin Road and either interchange. Austin Road will likely be relocated approximately 800 ft to the east when the interchange is constructed to remove the small radius reverse horizontal curves and to simplify the new interchange’s design and construction while maintaining the existing Austin Road intersection during construction. The relocation would increase the distance to SR 248 and decrease the distance to SR 56. These roads and interchanges are shown on the site map in Figure 1-1. Traffic Volumes and Characteristics US 32 currently has two lanes in each direction and a peak-hour volume of approximately 2,500 vehicles in each direction. The SR 248 interchange has ramp volumes of 400-600 vehicles during the peak hour. The SR 56 interchange has ramp volumes of 200-400 vehicles during the peak hour. The Austin Road interchange is forecast to have ramp volumes of 200-300 vehicles during the peak hour. Heavy vehicles account for 15% of the volume on the divided highway, and 5-10% of the volume on all interchanges, including the planned Austin Road interchange. Terrain in this area is level. The two existing interchanges are almost four miles apart (crossroad to crossroad) and traffic volumes are low. Placing a new interchange between the two existing interchanges will be physically feasible. 94 Guidelines for Ramp and Interchange Spacing

FIGURE 56 32 248 < 2 2 > At-grade intersection to be converted to a diamond interchange N CASE STUDY 1 SITE MAP 1-1

AGENCY REQUIREMENTS The state in which this project is located has an operating guideline of LOS C for rural multilane highways and freeways. The highway and all ramp- highway junctions in the project are currently operating at LOS C or better. The new interchange should not result in any components of the new freeway operating below LOS C. RAMP SP ACING CONSIDERA T IONS The following ramp spacing assessment follows the sequence outlined in Section 5.3: • Geometry considerations, • Traffic operations, • Safety, and • Signing. STEP 1—Geometric considerations: The first step is to conceptually determine the interchange footprint and approximate length of the ramps at the Austin Road interchange based on three-dimensional roadway design considerations . At a concept level and starting point in laying out diamond interchange ramps, physical entry and exit ramp gores are approximately 1,000 ft in length from the crossroad. T his distance generally meets vertical alignment needs for making appropriate grade changes and incorporating desired ramp geometry. On an entrance ramp, the distance between the physical gore and the painted merging tip typically varies from 400-800 ft based upon the horizontal curvature of the ramp and whether a taper or parallel entrance is used. On an exit ramp, the distance from the painted diverging tip to the physical gore typically varies from 300-500 ft for similar reasons. This results in the distance from a painted diverging tip to the crossroad generally ranging from 1,300-1,500 ft, and the distance from the crossroad to the painted merging tip generally ranging from 1,400-1,800 ft. These dimensions are further discussed in Chapter 3 of the Guidelines and shown in Exhibit 3-8. Ramp spacing dimensions under consideration in this case study (and all others) will be measured from the painted merging tip to the painted diverging tip. At this point in the planning process, no special conditions that could significantly impact ramp length have been identified at the Austin Road interchange. Dimensions similar to those noted above can be expected. Figure 1-2 shows approximate dimensions of all existing and proposed ramps A lthough ramp lengths cannot be determined until the pr o j ect enters the preliminary engineering phase, a range of approximate ramp lengths and resulting ramp spacin g v alues can be us ed to determine if issues related to traffic operations, signing, or safety are anticipated. in the project area, as well as centerline-to-centerline roadway spacing and 96 Guidelines for Ramp and Interchange Spacing

FIGURE LEGEND N CASE STUDY 1 RAMP SPACING AND TRAFFIC VOLUMES 1-2 9100’ 900’ 1400-1800’6400-6800’ spacing 1300-1500’ 8500-8700’ spacing 1400’ 1100’ 1300-1500’6500-6700’ spacing 8600-9000’ spacing 1800’ 800 800 300 200 200 200 300400? 2500 (1250/lane) ? 2500 (1250/lane) ? 2500 (1250/lane) ? 2500 (1250/lane) SR 24 8 A us tin R oa d (R e lo ca te d) SR 56 1100’ 900’ 400 500 11400’ 1400’ 80 0’’ A us tin R oa d (o ld a l ig nm en t) 1400-1800’ 300 1000’ 200 Note: Spacing defined from approximate location of merging and diverging painted tip

98 Guidelines for Ramp and Interchange Spacing design-hour traffic volumes. After these ramp configurations and spacing values have been developed from a geometric design perspective, the next steps are to consider the influence these spacing values may have on traffic operations, signing, safety, and other aspects. STEP 2—Traffic Operations: All ramp spacing dimensions between the existing interchanges and the proposed Austin Road interchange are greater than 6,000 ft, as depicted in the diagram in Figure 1-2. As discussed in Chapter 4 of the Guidelines, ramp spacings of this length have virtually no impact on traffic operations. If each ramp-freeway junction operates acceptably when analyzed in isolation, it is highly likely the entire freeway system will as well, and operations at any particular ramp-freeway junction will not be affected by adjacent ramps. The HCM provides basic capacity thresholds for various freeway and ramp components that are summarized in Table 4-1 of the Guidelines (reproduced below as Table 1-1). As shown in the table, the capacity of a merge on a two- lane (one direction) freeway is approximately 4,600 passenger cars per hour. At the Austin Road interchange, each of the ramp-freeway junctions will have a volume of approximately 2,700 vehicles per hour. Although a complete HCM analysis is needed to determine if the state agency’s operating guideline of LOS C is satisfied, the ramp-freeway junctions will clearly be below capacity and potentially meet the LOS guideline. Table 1-1 Approximate Capacity of Freeway-related Roadway Elements, 2010 HCM (5) Element Service Volume Freeway Lane 2,250 – 2,400 passenger cars per hour Single-Lane Ramp* 1,800 to 2,200 passenger cars per hour Merge Influence Area (on-ramp plus right two lanes of freeway) 4,600 passenger cars per hour Diverge Influence Area (off-ramp plus right two lanes of freeway) 4,400 passenger cars per hour * Basic ramp segment only, does not consider ramp terminal operations. Based on this conceptual-level evaluation, ramp spacing dimensions will not have a significant impact on traffic operations. Furthermore, each ramp- freeway junction will operate well below capacity. When this project enters the preliminary design phase, traffic operations should be reevaluated. However, no issues are anticipated. STEP 3—Safety: The safety consequences of an Austin Road interchange may be explored at a planning level without direct consideration of traffic volumes. Ramp spacing, The HCM does not have a procedure for analyzing an entrance ramp followed by an exit ramp on a two-lane freeway with no auxiliary lane. With ramp spacings of over 6,000 feet, though, operational impacts due to the spacing are unlikely. Although it was previously determined that spacing is unlikely to have an impact on freeway speed under these conditions, the impact of each ramp in isolation should also be considered. The HCM provides capacity thresholds summarized in Table 4-1 that can be used here.

measured from merging tip to diverging tip, ranges from 17,600 to 19,000 ft on the freeway segment between SR 248 and SR 56 without the Austin Road interchange. Ramp spacing values are estimated to range from 6,400 to 9,000 ft if the Austin Road interchange is constructed. All ramp combinations of interest consist of an entrance ramp followed by an exit ramp (EN-EX). Research conducted to develop the Guidelines indicated that the sensitivity of total crashes to EN-EX ramp spacing becomes close to negligible for spacing values greater than about 2,600 ft; in other words, the safety performance of the segment approaches that of a basic freeway segment with no interchange ramps. The finding is illustrated in Exhibit 5-5 (reproduced as Figure 1-3), where the solid line representing crash risk as a function of ramp spacing becomes fairly flat for spacing dimensions larger than 2,600 ft. 1 Relative crash risk is measured by the percent difference in crashes, of all types and severities, at some ramp spacing value compared to a ramp spacing of 1,600 ft . Figure 1-3 Preliminary Safety Assessment Tool for Ramp Spacing, Entrance Ramp Followed by Exit Ramp (Guidelines Exhibit 5-5) STEP 4—Signing: Signing should be considered at the earliest stages of concept development to assess the types and amount of information that will need to be presented and to consider the advance placement of signs. It may be necessary to consider more than one upstream and one downstream interchange because advance guide signs may be placed several miles before an interchange. At Section 4.5.4.1.1, Section 5.3.3.1, and Exhibit 5-5 of the Guidelines show that negligible differences in freeway mainline safety are expected between two ramp spacing dimensions that are both significantly greater than 2600 feet. Appendix A 99

this location, there are no interchanges within four miles west of SR 248 or two miles east of SR 56. The area depicted in Figure 1-1 is effectively isolated from a signing perspective. All three of these interchanges have a ramp ADT of more than 100 vehicles, and thus are classified by the MUTCD as “major” or “intermediate” interchanges. At major and intermediate interchanges, the MUTCD recommends an advance guide sign be placed ½ mile and one mile in advance of an exit, with a third guide sign placed two miles in advance of the exit if spacing permits. There are currently advance guide signs for SR 248 two miles prior to the exit in both directions, and the state wishes to maintain these two-mile advance signs due to the importance of SR 248 to its transportation network. Austin Road and SR 56 are not highways of regional importance or facilities where a high number of drivers unfamiliar with the area are expected. Therefore, this planning-level analysis will only consider the placement of advance guide signs ½ mile and one mile in advance of the Austin Road and SR 56 interchanges. If advance guide signs two miles prior to one or more of the interchanges is desired, a more detailed analysis can be completed at a later stage of the project. Placement of such signs would be optional. Figure 1-4 shows a sign placement concept for the westbound direction of the freeway. Guide signs are placed one mile and ½ mile in advance of each offramp, as well as at the offramp itself. This figure depicts existing signs and new signs associated with the proposed interchange. The Austin Road exit gore will be located at approximately the same location as the existing two- mile advance guide sign for the SR 248 interchange. When close spacing of signs occurs, there are several signing options: • The SR 248 advance guide sign and the Austin Road exit sign could be placed together on an overhead assembly. However, the MUTCD discourages the placement of other signs with an exit sign, so this is not recommended. • Interchange sequence signs could be used. The MUTCD only recommends these signs in urban areas where multiple interchanges are spaced closely together. US 32 does not meet these criteria, so such signs are not recommended. • The SR 248 advance guide sign could be moved downstream from the Austin Road exit. Placing the sign 1 ¾ miles in advance of the SR 248 exit would still provide advance notice of the exit to drivers, and is recommended. 100 Guidelines for Ramp and Interchange Spacing

Overall, signing needs for US 32 in the westbound direction will not place more than two sign panels at the same location or create special conditions that would require an unusually high number of message units. For brevity, signing on US 32 eastbound is not illustrated in the case study but will be very similar to the westbound direction and will not affect the feasibility of the Austin Road interchange. Signing principles of the MUTCD can be attained with the new Austin Road interchange. Signs related to these interchanges will not present drivers with more information than they are able to process; therefore, signing needs do not affect ramp spacing considerations. FINDINGS At the first stage of the concept development, a diamond interchange at Austin Road appears to be feasible from a ramp and interchange spacing perspective. Based on forecast ramp and freeway volumes and anticipated ramp spacing dimensions, no components of the freeway will be over capacity and the LOS C guideline is potentially achievable. Signing needs at this location appear consistent with MUTCD principles for sign placement and sign information content. At no location is more than one guide sign necessary. There is no expected reduction in safety along the freeway mainline after adding a diamond interchange at Austin Road because ramp spacing dimensions remain significantly larger than 2,600 ft. Ramp and interchange spacing considerations should be reevaluated as the concept developed enters preliminary design, although no issues are anticipated. Traffic operations should be refined and documented in the interchange evaluation document for this project. Although one existing sign will need to be moved, the new interchange can easily be signed without presenting drivers with an excessive number of message units. 102 Guidelines for Ramp and Interchange Spacing

Case Study 2 Case Study 2 introduces several conditions not found in Case Study 1. The project includes partial interchanges, and lies on the Interstate Highway System. The proposed interchange will create ramp spacing that is close enough to constitute a weaving section. BACKGROUND General A new employment center is proposed in a historically rural community that is becoming suburbanized. The proposed site is located between Interstate 50 and a lake. A railroad runs parallel to the north side of the interstate (between the interstate and the lake), and a steep hillside and power line are on the south side of the interstate. The employment center will generate more traffic than the existing roadway network can accommodate, and a new diamond interchange is proposed at the existing Jefferson Road overpass on I-50. A diamond interchange will have a relatively narrow footprint compared to partial cloverleaf forms and will minimize impacts in this constrained site. Adjacent Interchanges To the east, the nearest interchange is at Adams Road, 5,100 ft away from Jefferson Road (measured between the centerlines of each crossroad). Adams Road has a half-diamond interchange, with the ramps on the east side away from Jefferson Road. To the west, the nearest interchange is Main Street, 6,900 ft away from Jefferson Road. Main Street also has a half-diamond interchange, with the ramps on the east side towards Jefferson Road. These roads and interchanges are shown on the site map in Figure 2-1. Traffic Volumes and Characteristics The freeway currently has two lanes in each direction and a peak-hour volume of approximately 2,500 vehicles in each direction. The Adams Road interchange has a peak-hour volume under 200 vehicles on each ramp. The Main Street interchange has a peak-hour volume of 400 to 600 vehicles on each ramp. When the employment center is fully built out, the Jefferson Road interchange is expected to have 500 peak-hour vehicles on the ramps to the east and 700 peak-hour vehicles on the ramps to the west. Heavy vehicles account for 10% of the interstate volume and less than 5% of the volumes on the existing ramps. The heavy-vehicle percentage on the ramps at the new interchange is forecast to be 5%. The interstate is generally level through this area, although there is a steep uphill slope on the south side. Appendix A 103

FIGURE N R A I L R O A D J e f f e r s o n R o d a EMPLOYMENT CENTER Slope Down Slope Down E N I L R E W O P t e e r t S n i a M Lake < 2 2 > CASE STUDY 2 SITE MAP 2-1 Proposed New Interchange Source: The U.S. Department of Agriculture, Service Center Agencies

AGENCY REQUIREMENTS The state in which the project is located has an operating guideline of LOS D for this type of location. The interstate and all ramp-freeway junctions in the project area are currently operating at LOS D or better, and the new interchange should not result in any components of the freeway operating below LOS D. RAMP SPACING CONSIDERATIONS The following ramp spacing assessment follows the sequence outlined in Section 5.3: • Geometry considerations, • Traffic operations, • Safety, and • Signing. STEP 1—Geometric considerations: The first step is to conceptually determine the interchange footprint and approximate length of the ramps at the Jefferson Road interchange based on three-dimensional roadway design considerations. As discussed in Chapter 3 of the Guidelines and in Case Study 1, at a conceptual level and starting point in layout for diamond interchange ramps, physical entry and exit gores are approximately 100 ft in length from the crossroad. On an entrance ramp, the distance between the physical gore and the painted merging tip is typically 400-800 ft and, on an exit ramp, the distance between the painted diverging tip and the physical gore is typically 300-500 ft. Summing these dimensions, the distance from a painted diverging tip to the crossroad generally ranges from 1,300-1,500 ft, and the distance from the crossroad to the painted merging tip generally ranges from 1,400- 1,800 ft. Considering the site specific design needs at the Jefferson Road interchange, the length of the eastbound ramps may be impacted by the steep uphill slope on the south side of the interstate. An overpass is generally in the range of about 25 ft above a freeway, but the southern end of the Jefferson Street overpass may be closer to 40 ft above the elevation of I-50. This means the eastbound off- and onramps will need to be lengthened to accommodate the needed grade changes. Assuming a 4-6% grade and an additional 15 ft of elevation change, approximately the assumed distance between the cross street and the painted tips of the eastbound ramps should be lengthened by an additional 300-500 ft. As discussed in Chapter 1, ramp spacing dimensions under consideration in the case studies will be measured from the painted merging tip to the painted diverging tip. Appendix A 105

FIGURE N CASE STUDY 2 RAMP SPACING AND DAILY TRAFFIC VOLUMES 2-2 Note: Spacing defined from approximate location of merging and diverging painted tip 6400’ 5100’ 1000’ 1400-1800’4100-4500’ spacing 1300-1500’ 3600-3800’ 1400’ 1500’ 1600-2000’3400-3800’ spacing 1700-2300’ 2800-3400’ 1100’ 00 4 006 005 005 007 007 002 re dnu 002rednu ~2500 (1250/lane) ~2500 (1250/lane) ~2700 (1350/lane) ~2700 (1350/lane) t e er t S ni a M da o R n o sr ef f e J da o R s m a d A 5000-5200’ spacing 3900-4500’ spacing

Figure 2-2 shows approximate dimensions of all existing and proposed ramps in the project area as well as centerline-to-centerline roadway spacing and design-hour traffic volumes. Note that ramps in the eastbound direction are to accommodate a greater grade change between the interstate and Jefferson Road. After considering the ramp configurations and spacing values from a geometric design perspective, the next steps are to consider the potential influence that traffic operations, signing, and other considerations have on ramp spacing values. STEP 2—Traffic Operations: The Jefferson Road interchange will create the following four, closely spaced ramp combinations: • Eastbound, upstream of Jefferson Road—Entry ramp (600 vehicles per hour (vph)) followed by exit ramp (700 vph). Ramp spacing of 3,400-3,800 ft. • Eastbound, downstream of Jefferson Road—Entry ramp (500 vph) followed by entry ramp (under 200 vph). Ramp spacing of 3,900- 4,500 ft. • Westbound, upstream of Jefferson Road—Exit ramp (under 200 vph) followed by exit ramp (500 vph). Ramp spacing of 5,000-5,200 ft. • Westbound, downstream of Jefferson Road—Entry ramp (700 vph) followed by exit ramp (400 vph). Ramp spacing of 4,100 to 4,500 ft. There is little guidance available on operational analysis of closely spaced exit-exit or entry-entry ramp combinations. Exit-exit ramp combinations do not result in any vehicles entering the freeway, an act that is more disruptive to traffic flow than vehicles leaving the freeway. With a spacing of 5,000 ft or more between the Adams Road and Jefferson Road exit ramps on I-50 westbound, it is unlikely that any operational impacts will occur due to spacing. As discussed in the Section 4.3.1 of the Guidelines, simulation modeling has indicated that the spacing of entry-entry ramp combinations does not have a significant impact on freeway speed when the freeway volume is 1,500 vehicles per hour per lane or less. Volumes on I-50 eastbound are not this high, making it unlikely that the Jefferson Road and Adams Road entry ramps will create operational impacts on the freeway. In addition to checking operational impacts due to spacing, each ramp - freeway junction should be evaluated to see if, in isolation, it is near or under capacity. As discussed in Case Study 1, Table 4-1 of the Guidelines summarizes the HCM’s capacity thresholds for various freeway and ramp Simulation results suggest the spacing of entry-entry ramps, with the volumes that will exist, will not have an impact on freeway speed. expected to be 300-500 ft longer than ramps in the westbound direction Appendix A 107

components. The table shows that all ramp-freeway junctions on this segment of I-50 appear to be well below the capacity thresholds of the HCM. At a concept level, this indicates there are no obvious pronounced traffic operations concerns. Detailed traffic operations should be conducted as the geometry is being refined. On I-50 between Main Street and Jefferson Road, a closely spaced entry-exit ramp combination will exist in both directions. Auxiliary lanes are being considered between each entry and exit ramp, which would create a weaving section. The AASHTO Green Book recommends auxiliary lanes to improve traffic operation between a successive entrance and exit terminal when the spacing between the ramp “noses” is 1,500 ft or less. The use of auxiliary lanes is not discouraged if greater ramp spacings exist, as is the case here. However, simulation modeling suggests that the benefits of an auxiliary lane may be limited at the Jefferson Road interchange. In each direction of I-50, less than 1,250 vehicles per hour per lane will be on the freeway upstream of each entry ramp. Simulation modeling of entry-exit ramp combinations with 1,250 vehicles per hour per lane on the freeway found that ramp spacing had no impact on freeway speed. Simulation modeling was only conducted with a four-lane (in one direction) freeway, so the findings should be used with caution. Subsequently, in the next phase of the project, a complete HCM ramp- freeway junction analysis should be conducted to determine the LOS. If an auxiliary lane is added, the resulting segment will either be considered, for the purposes of HCM analysis, a weaving section or a basic freeway segment. To make this determination, Exhibit 4-5 of the Guidelines may be used. Use of this exhibit requires only three pieces of data–the ramp spacing (weaving segment length), ratio of weaving volume to total volume, and the number of lane changes to complete a weaving maneuver. The eastbound direction of I-50 has a shorter ramp spacing dimension, the same freeway volume, and higher ramp volumes relative to the westbound direction of I-50. Therefore, the eastbound direction is the focus of the analysis presented below; similar analysis should be conducted for the westbound direction. At this phase of the project, the origin and destination of vehicles in the weaving segment is not known. To account for worst-case conditions, it is assumed that all vehicles on the ramps make weaving maneuvers (there are no vehicles that enter at the first ramp and exit at the second ramp). Under such a scenario, there would be 1,300 (600 + 700) weaving vehicles and the ratio of weaving volume to total volume would be 0.48 (1,300/2,700). Using Exhibit 4-5 of the Guidelines (reproduced and marked up here as Figure 2-3), the Main Street entry ramp and the Jefferson Road exit ramp will be “close enough” together and have a “high enough” ratio of weaving to non- No operational guidelines or warrants for the use of auxiliary lanes exist. However, simulation model results suggest the benefits of adding one here may be limited. 108 Guidelines for Ramp and Interchange Spacing

weaving volume that the weaving procedures of the HCM should be used for operational analysis. Figure 2-3 Determination of Analysis Procedure for I-50 Eastbound Between Main Street and Jefferson Road. As shown in Figure 2-3, the section of I-50 between Main Street and Jefferson Road will fall into the realm of weaving, as defined by the HCM, if an auxiliary lane is added. However, this does not imply that operational problems will occur or a desired LOS cannot be achieved. Rather, it implies that the weaving procedures of the HCM, as opposed to the basic freeway segment procedures, should be used to analyze the section. In summary, the current configuration and weaving screening indicates the interchange should be evaluated as a weaving section (if an auxiliary lane is ultimately used), and this revelation can help guide the scoping for an appropriate range of traffic analyses as the interchange concepts are developed in more detail. STEP 3—Safety: The Jefferson Road interchange will create four ramp combinations of interest. The combinations, ramp spacing values, and applicable sections of the Guidelines for the safety assessment are summarized as the following: • Eastbound, upstream of Jefferson Road—Entry ramp followed by exit ramp; ramp spacing of 3,400-3,800 ft; Sections 4.5.4.1 and 5.3.3.1. • Eastbound, downstream of Jefferson Road—Entry ramp followed by entry ramp; ramp spacing of 3,900-4,500 ft; Sections 4.5.4.2 and 5.3.3.2. • Westbound, upstream of Jefferson Road—Exit ramp followed by exit ramp; ramp spacing of 5,000-5,200 ft; Section 4.5.4.3. Freeway mainline safety issues are not expected between entrance and exit ramps in either the eastbound or westbound directions following construction of the Jefferson Road interchange. Appendix A 109

• Westbound, downstream of Jefferson Road—Entry ramp followed by exit ramp; ramp spacing of 4,100 to 4,500 ft; Sections 4.5.4.1 and 5.3.3.1. The safety consequences of the Jefferson Road interchange can be explored at a planning level in this case study without direct consideration of traffic volumes. The ramp spacing for the EN-EX combinations will be 3,400- 3,800 ft in the eastbound direction and 4,100-4,500 ft in the westbound direction. Research conducted to develop the Guidelines indicated that the sensitivity of total crashes to EN-EX ramp spacing becomes close to negligible for spacing values greater than about 2,600 ft; in other words, the safety performance of the segment approaches that of a basic freeway segment with no interchange ramps. The finding is illustrated in Exhibit 5-5, where the solid line representing crash risk as a function of ramp spacing becomes fairly flat for spacing dimensions larger than 2,600 ft. The ramp spacing for the eastbound EN-EN combination will range from 3,900-4,500 ft. Research conducted to develop the Guidelines indicated that the sensitivity of total crashes to EN-EN ramp spacing becomes close to negligible for spacing values greater than about 2,200 ft. The finding is illustrated in Exhibit 5-8, where the solid line representing crash risk as a function of ramp spacing becomes fairly flat for spacing dimensions larger than 2200 ft. The ramp spacing for the westbound EX-EX combination will range from perspective during research conducted to develop the Guidelines. STEP 4—Signing: Signing should be considered at the earliest stages of concept development to assess the types and amount of information that will need to be presented and to consider the advance placement of signs. It may be necessary to consider more than one upstream and one downstream interchange because advance guide signs may be placed several miles prior to an interchange. At this location, there are no interchanges within three miles west of Main Street or east of Adams Road. The area depicted in Figure 2-1 is effectively isolated from a signing perspective. The existing interchanges and the proposed Jefferson Road interchange are all considered “major” or “intermediate” by the MUTCD. Advance guide signs should be placed ½ and one mile prior to each exit, with a third sign two miles in advance of the exit being optional. The Main Street and Adams Road interchanges do not currently have 2-mile advance guide signs, and such signs are not planned for the Jefferson Road interchange. Freeway mainline safety issues are also not expected between the consecutive entrance ramps in the eastbound direction following construction of the Jefferson Road interchange. Freeway mainline safety issues for an EX-EX spacing between 5,000- 5,200 feet are not likely given the relatively low volumes at the ramp- freeway junctions. The geometric analysis and signing considerations are the primary factors for the EX-EX spacing assessment. 5,000-5,200 ft. The EX-EX combination was not studied from a safety 110 Guidelines for Ramp and Interchange Spacing

Figure 2-4 shows a sign placement concept for the westbound direction of the freeway. In the eastbound direction, there is only one exit ramp, and signing requirements will clearly be achievable. As shown in Figure 2-4, the guide sign one mile in advance of the Main Street offramp is presently within a few hundred feet of the Jefferson Road diverging tip, at approximately the same location where a sign for the Jefferson Road offramp will need to be placed. There is limited flexibility with the location of the sign for the Jefferson Road offramp. However, the advance guide sign for Main Street could be placed at a location other than one mile in advance of the exit. The sign could be mounted on the Jefferson Road overpass ¾ mile prior to the Main Street exit, or it could be placed 1 ½ miles prior to the Main Street exit (at the same location as the ½ mile advance guide sign for the Jefferson Road exit). If the latter is chosen, an overhead sign structure would be needed. Both options can be explored in greater detail in the preliminary design phase of the project. However, it is clear that signing principles of the MUTCD can be satisfied, and no further analysis is necessary at this time. Signs related to these interchanges will not present drivers with more information than they are able to process; therefore, signing needs do not affect ramp spacing considerations. Other Considerations: The proposed interchange will be on the Interstate Highway System, and therefore must be approved by FHWA. As discussed in Chapter 2 of the Guidelines, FHWA considers eight points before granting or denying access. These points address the following issues: 1. The existing system is incapable of accommodating traffic demands; 2. All reasonable alternatives to a new interchange have been considered; 3. The proposal does not adversely impact the freeway; 4. A full interchange at a public road is provided; 5. The proposal is consistent with transportation plans; 6. A comprehensive interstate network study is prepared; 7. There is coordination with transportation system improvements; and, 8. The request needs to consider planning and environmental constraints. The initial ramp spacing analysis conducted above will, in part, determine whether the third point is satisfied. Appendix A 111

FINDINGS At the first stage of the concept development, a diamond interchange at Jefferson Road appears to be feasible from a ramp and interchange spacing perspective. Based on forecast ramp and freeway volumes and anticipated ramp spacing dimensions, the LOS D guideline for this facility appears achievable. Signing needs at this location appear consistent with MUTCD principles for sign placement and sign information content. At no location is more than one guide sign necessary. Safety is relatively insensitive to ramp spacing within the ranges expected for the EN-EX, EN-EN, and EX-EX combinations following construction of the Jefferson Road interchange. Ramp and interchange spacing considerations should be reevaluated as the concept developed enters preliminary design, and traffic operations analyses should be performed as the concepts are being refined. Appendix A 113

Case Study 3 Case Study 3 presents a project where a proposed interchange will be one mile from adjacent interchanges in either direction. The proposed interchange is likely geometrically feasible, but the presence of high traffic volume creates operational and safety concerns. A preliminary analysis is conducted in this case study, and the need for additional analyses as the design is refined is highlighted. BACKGROUND General An eight-lane interstate (I-121) runs through a built-out suburban area where arterials are spaced one mile apart. To improve the transportation system, the state transportation agency is proposing a new interchange at 44th Street, one of the arterial streets on the one-mile grid. When the interstate was initially constructed, no interchange was constructed at 44th Street. A single-point diamond interchange is considered the most feasible interchange form for this location because it has a small footprint and is consistent with the other interchanges in the corridor. The state is considering adding auxiliary lanes between the ramps for the 44th Street interchange and ramps from adjacent interchanges. Auxiliary lanes have been used at some other locations in the I-121 corridor to improve operational performance. Adjacent Interchanges To the north, the nearest interchange is at 48th Street, 5,300 ft away from 44th Street (measured between the centerlines of each crossroad). To the south, the nearest interchange is at 40th Street, 5,400 ft away. Both interchanges are single-point diamonds. These roads and interchanges are shown on the site map in Figure 3-1. Traffic Volumes and Characteristics I-121 currently has four lanes in each direction. The interstate is primarily used by commuters, and experiences a heavy directional split during the peak periods. Conditions during the p.m. peak, when overall volume is highest, are described below. During the p.m. peak, 8,000 vehicles (approximately 2,000 per lane) are travelling northbound on I-121 south of the 40th Street interchange. The 40th Street, 44th Street, and 48th Street interchanges will have 700-1,200 northbound vehicles exiting and 300-500 entering northbound vehicles. North of the 48th Street interchange, there will be 6,200 northbound vehicles on I-121. Heavy vehicles account for less than 5% of volume on the freeway and surrounding arterials, and terrain in this area is level. Volumes in the southbound direction, which are lower during this period, are shown in Figure 3-2. Arterials in this area were built on a one-mile grid. Exhibit 5-2 indicates that, geometrically, the diamond forms can fit within the cross street spacing available at this site. For brevity, only one time period is analyzed within this case study. However, the a.m. peak hour and any other high-volume periods known to exist should also be analyzed. 114 Guidelines for Ramp and Interchange Spacing

FIGURE (NO SCALE) N <4 4 > 121 Proposed New Interchange CASE STUDY 3 SITE MAP 3-1 Reprinted with permission from The Sanborn Map Company, Inc. © The Sanborn Map Company, Inc., 2011. All rights reserved. Appendix A 115

FIGURE (NO SCALE) N Note: Spacing defined from approximate location of merging and diverging painted tip 3-2 CASE STUDY 3 RAMP SPACING AND TRAFFIC VOLUMES ’ 00 35 ’ ’0002 ’0002 ’0061 -0031 g ni c ap s ’0061 -0031 g ni c ap s ’0002 - ’0071 ’0002 - ’0071 ’0081 -0041 g ni c ap s ’0081 -0041 g ni c ap s ’0091 ’0091 0037 ) e n al/5281(0065 ) e n al/0041( 0096 ) e n al/5271(0065 ) e n al/0041( 48th Street 44th Street 40th Street ’0091 ’0091 ’ 00 45 ’ ’0002 ’0002 ’0002 - ’0061 ’0002 - ’0061 ’0071 ’0071 ’0002 -0061 g ni c ap s ’0002 -0061 g ni c ap s ’0002 - ’0061 ’0002 - ’0061 ’0012 -0081 g ni c ap s ’0012 -0081 g ni c ap s ’0091 ’0091 ’0071 ’0071 ’0061 ’0061 ’0002 - ’0071 ’0002 - ’0071 0001 004 003 006 007 003 003 003 0021 004 005 005 0008 ) e n al/0002(0055 ) e n al/5731( 0026 ) e n al/0541(0085 ) e n al/0541( 116 Guidelines for Ramp and Interchange Spacing

AGENCY REQUIREMENTS The state in which this project is located has an operating guideline of LOS E for urban interstates. Additionally, state maintains the traffic signals at all ramp-terminal intersections and has an operating standard of LOS D for signalized intersections. The interstate and the existing ramp-terminal intersections are currently meeting these standards. The new interchange should not result in any components of the new freeway operating below LOS E or any ramp-terminal intersection operating below LOS D. RAMP SPACING CONSIDERATIONS The following ramp spacing assessment follows the sequence outlined in Section 5.3: • Geometry considerations, • Traffic operations, • Safety, and • Signing. STEP 1—Geometric Considerations: The first step is to conceptually determine the interchange footprint and approximate length of the ramps at the 44th Street interchange based on three-dimensional roadway design consideration. Conditions at the 44th Street interchange will be similar to those at the two adjacent interchanges. All interchanges will: • Be single-point diamond form; • Serve arterials with similar volumes of traffic as the adjacent interchanges; • Have a transition from a single lane to multiple lanes on exit ramps and vice versa on entry ramps; • Have metered entry ramps; • Have the potential for long queues on the exit ramps; and • Use the state’s standard gore design. Measured from the crossroad to the painted gore, the 40th and 48th Street interchanges have exit ramps that vary from 1,600-2,000 ft and entry ramps that vary from 1,700-2,000 ft. Since the design of the 44th Street interchange will be similar to these interchanges, these ranges of ramp lengths are used at the conceptual planning level of the 44th Street interchange. Ramp lengths are shown in Figure 3-2. On the 44th Street exit ramps, queues of several hundred feet in length can be anticipated during the p.m. peak hour. The ramp should be designed so that drivers can decelerate from the speed of the freeway to a complete stop by Analysis of ramp-terminal intersections will not be included in the conceptual evaluation presented in this case study but should be considered at the next phase of the project. At the 44th Street interchange, the design of exit ramps will be heavily influenced by ramp- terminal intersection queues, and the design of entrance ramps will be heavily influenced by ramp meter queues. Appendix A 117

the time the back of queue is reached. The FFS of the freeway can be used for a conservative design. However, if freeway volumes are high enough to create congestion at the same time peak queues are expected, a speed lower than FFS could be used when determining deceleration distance. Ultimately, queue considerations will likely dictate a ramp length that is greater than required due to grade change alone. A similar situation exists on the entrance ramp. The ramp meter will need to be placed far enough down the ramps that queues will not spill back onto 44th Street. After stopping at the meter, drivers will then need sufficient ramp length to accelerate to nearly the speed of the freeway onto which they are merging. Queue length and acceleration/deceleration length calculations should be performed during the project’s preliminary design phase to determine actual ramp length dimensions. At this stage of the project, it is assumed the dimensions needed to achieve these characteristics at the 44th Street ramps will be similar to the ranges of dimensions that exist at the ramps at the adjacent interchanges. After considering the ramp configurations and spacing values from a geometric design perspective, the next steps are to consider the potential influence that traffic operations, signing, and other considerations have on ramp spacing values. STEP 2—Traffic Operations: The 44th Street interchange will introduce four ramp spacings of approximately 2,000 ft or less. AASHTO policy recommends auxiliary lanes when the distance between successive “ramp noses” is less than 1,500 ft, and this may be the case here. Simulation models of a four-lane freeway with 1,750 vehicles per hour per lane (the approximate volume on I-121 at 44th Street) have identified a freeway speed reduction of up to 20 mph when an auxiliary lane is not present between closely spaced ramps. However, under the range of ramp volumes at these interchanges, the expected speed reduction due to the lack of an auxiliary lane would be less than 5 mph. Auxiliary lanes are recommended between all four of the EN-EX ramp combinations that will be created. Ramp spacing dimensions in the range of 1,500-2,000 ft are short enough that, when an auxiliary lane is added between the ramps, a weaving section will be created regardless of what volumes are present, and an HCM weaving analysis should be conducted. The weaving section will be considered one- sided, since both ramps will be on the same (right) side of the freeway. The one-sided weaving sections that will be created here may be designed in one of two ways. If designed as a ramp weaving, a lane will be added from a single-lane entry ramp, carried through the section, and dropped at a single- lane exit ramp. Designed as a major weave, a lane will be added from a single-lane entry ramp, carried through the section, and dropped at a double- lane ramp (where the second lane comes from a taper off of one of the Precise dimensions of the 44th Street ramps will be determined during the project’s preliminary design phase. Approximate ranges of dimensions based upon the design of adjacent interchanges may be used at this stage of the project to make an initial assessment of the adequacy of ramp spacing. A complete weaving analysis for all four weaving sections being created by the 44th Street interchange should be conducted as the design is developed to a level of detail that better quantifies ramp length dimensions. 118 Guidelines for Ramp and Interchange Spacing

freeway’s basic lanes). These two options are illustrated in Exhibit 12-3 of the 2010 HCM (reproduced as Figure 3-3) Ramp weave Major weave with two-lane exit ramp Figure 3-3 Types of One-Sided Weaving Segments (Reproduced from 2010 HCM). Two-lane exit ramps will maintain the principles of lane balance. The number of lanes downstream of the diverge (on the freeway and the ramp combined) is one more than the number of lanes on the freeway prior to the diverge. Exiting vehicles coming from the freeway are not required to make a lane change, which will improve the operation of the weaving section. An HCM analysis can be used to quantify this improvement. However, two-lane exit ramps may not be required to achieve capacity and/or desired LOS. For weaving segments where the exit ramp already exists (i.e., a 40th Street or 48th Street exit), converting existing single-lane ramps into double-lane ramps will increase the scope and extent of this project. STEP 3—Safety: spacing dimensions range from 6,000 to 7,400 ft without a 44th Street interchange; all ramp combinations of interest are EN-EX. Guidelines Exhibit 5-5 (reproduced below as Figure 3-4) indicates that spacing dimensions in this range generally correspond to safety performance of a freeway segment without interchanges. The relative crash risk “levels off” to about -12% for spacing dimensions beyond 3,000 ft. Ramp weaves with single- lane exits may be adequate for all weaving sections created by the 44th Street interchange. If they are not, major weaves with double-lane exits should be considered. The safety consequences of a 44th street interchange may first be explored at a planning level without explicit consideration of traffic volumes. Ramp Appendix A 119

1 Relative crash risk is measured by the percent difference in crashes, of all types and severities, at some ramp spacing value compared to a ramp spacing of 1,600 ft Figure 3-4 Preliminary Safety Assessment Tool for Ramp Spacing, Entrance Ramp Followed by Exit Ramp (Guidelines Exhibit 5-5) Ramp spacing dimensions range from 1,300 to 2,100 ft with an interchange at 44th Street. Again, all ramp combinations of interest are EN-EX. Figure 3-4 shows a relative crash risk of about -6% for a spacing of 2,100 ft; +7% for a 1,300 ft spacing. Therefore, the expected number of crashes along the freeway mainline is estimated to increase by anywhere from 6 to 19% with the 44th Street interchange in place. These estimates assume all else is equal, in Guidelines Section 4.5.4.1.4 indicates the expected increase in crashes can be reduced, or possibly eliminated, if auxiliary lanes are provided between entrance and exit ramps. Traffic patterns are likely to change following construction of the 44th Street interchange. In addition, safety impacts of adding auxiliary lanes between the ramps for the 44th Street interchange and ramps from adjacent interchanges Exhibit 5-7 is illustrated next. The total number of crashes expected to occur can be estimated with Guidelines Equation 5-1: including traffic volumes, and that no auxiliary lanes are used. Discussion need to be assessed in greater detail. A fuller safety assessment that addresses these issues by implementing Guidelines Equation 5-1 and Guidelines 120 Guidelines for Ramp and Interchange Spacing

Guidelines Equation 5-1: Estimating the total number of crashes between an entrance and exit Equation variables are defined in Guidelines Section 5.3.3.1. Applications to the northbound direction of I-121 are demonstrated in this case study. A safety analysis of the southbound direction can be conducted using the same basic steps. L represents the segment length, measured in miles. An analysis segment is defined from physical gore to physical gore. If exact locations of the physical gore are unknown, the analysis segment may be defined from cross street to cross street. The segment length without the 44th street interchange is approximately 2 miles (the distance between 48th and 40th streets). ADTEN and ADTEX represent the daily volumes of cars entering and exiting on the analysis segment. The volumes without the 44th street interchange are shown in Figure 3-5. In the northbound direction, these numbers are 5,400 and 11,250 vehicles per day, respectively. DADT is the daily volume on the freeway mainline upstream of the entrance gore in the analysis direction. This number for the northbound segment without the 44th street interchange can be determined from Figure 3-6 as 67,000 – 13,000 = 54,000 vehicles per day. S is the ramp spacing in feet, defined from painted merging tip to painted diverging tip. The spacing between the merging tip (from 40th street) and diverging tip (to 48th street) is 6,700 ft in the northbound direction (see Figure 3-5). No auxiliary lane is present in the current condition, so the variable ‘AuxLn’ is set to zero. The total number of crashes expected on the northbound freeway mainline between 40th street and 48th street without a 44th street interchange is: ( ) ( ) ( ) ×−×= − AuxLn S ADTADTDADTLTOTAL EXEN 23.0 450 exp107.9 02.018.012.10.16 ( ) ( ) ( ) yrcrashesTOTAL /23023.0 6700 450 exp112505400540002107.9 02.018.012.10.16 ≈×−×= − Guidelines Exhibit 5-7 (reproduced as Figure 3-7) shows that the percentage of crashes expected to result in a fatality or injury to at least one vehicle occupant levels off at 30% for ramp spacing values greater than about 1,800 ft. Therefore, 23 x 0.30 = 7 crashes per year are expected to be fatal plus injury on the northbound freeway mainline between 40th Street and 48th without the 44th Street interchange. Appendix A 121

FIGURE (NO SCALE) N 3-5 CASE STUDY 3 RAMP SPACING AND DAILY TRAFFIC VOLUMES WITHOUT 44TH STREET INTERCHANGE 6, 70 0' sp a ci n g 6, 70 0' sp ac in g 7, 40 0' sp a ci n g 7, 40 0' sp ac in g 10 , 70 0' 10 ,7 00 ' 48th Street 40th Street 11 , 25 0 5, 40 0 4, 60 0 5, 40 0 3, 75 0 13 ,0 00 6, 30 0 4, 60 0 67 ,0 00 48 ,0 00 46 ,3 00 48 ,6 00 Note: Spacing defined from approximate location of merging and diverging painted tip 122 Guidelines for Ramp and Interchange Spacing

FIGURE (NO SCALE) N 3-6 CASE STUDY 3 RAMP SPACING AND DAILY TRAFFIC VOLUMES WITH 44TH STREET INTERCHANGE 5, 30 0' 5, 30 0' 5, 40 0' 5, 40 0' 1, 80 0' – 2, 10 0' sp ac in g 1, 80 0' – 2, 10 0' sp ac in g 1, 60 0' – 2, 00 0' sp ac in g 1, 60 0' – 2, 00 0' sp ac in g 1, 40 0' – 1, 80 0' sp ac in g 1, 40 0' – 1, 80 0' sp ac in g 1, 30 0' – 1, 60 0' sp ac in g 1, 30 0' – 1, 60 0' sp ac in g 48th Street 40th Street 8, 35 0 4, 20 0 3, 30 0 4, 20 0 2, 50 0 10 ,2 00 5, 00 0 3, 30 0 44th Street 5, 85 0 2, 50 0 2, 50 0 2, 50 0 67 ,0 00 48 ,0 00 46 ,3 00 46 ,3 00 57 ,9 00 61 ,2 00 Note: Spacing defined from approximate location of merging and diverging painted tip Appendix A 123

Figure 3-7 Crash Type and Severity Distributions as a Function of Ramp Spacing (Guidelines Exhibit 5-7) Safety analysis in the northbound direction with the 44th street interchanges requires defining two analysis segments: Segment 1: From 40th Street to 44th Street, Northbound ‘L’ = 5400/5280 1 mile ‘ADTEN’ and ‘ADTEX’ = 4200 and 5850 vehicles per day, respectively DADT = 67000 – 10200 = 56800 vehicles per day ‘S’ = 1600 ft (average of expected range of 1400-1800 ft) AuxLn = 0 if no auxiliary lane; 1 if auxiliary lane present Figure 3-7 shows about 29% of crashes are expected to be fatal plus injury for a 1600 ft spacing. Segment 2: From 44th Street to 48th Street, Northbound ‘L’ = 5300/5280 1 mile ‘ADTEN’ and ‘ADTEX’ = 2500 and 8350 vehicles per day, respectively Applying Guidelines Equation 5-1 to Segment 1, the total expected number of crashes on the northbound freeway mainline between 40th street and 44th street is 14 crashes per year without an auxiliary lane, 12 crashes per year with an auxiliary lane. Approximately 4 crashes per year are expected to be fatal plus injury on the northbound freeway mainline between 40th street and 44th street without an auxiliary lane, approximately 3 crashes per year with an auxiliary lane. 124 Guidelines for Ramp and Interchange Spacing

DADT = 61200 – 5850 = 55350 vehicles per day ‘S’ = 1,450 ft (average of expected range of 1,300-1,600 ft) AuxLn = 0 if no auxiliary lane; 1 if auxiliary lane present Figure 3-7 shows about 28% of crashes are expected to be fatal plus injury for a 1,450 ft spacing. Comparisons of possible scenarios to the current condition are summarized below in Table 3-1. Table 3-1 Expected Change in Safety Performance with addition of 44th Street Interchange Scenario Expected Change in Mainline Safety compared to ‘No Build’ (i.e., no 44th street interchange) No Build --- 44th street interchange; no auxiliary lanes 17% increase in total crashes; 14% increase in fatal plus injury crashes 44th street interchange; auxiliary lane between 40th and 44th street ramps only 44th street interchange; auxiliary lane between 44th and 48th street ramps only 9% increase in total crashes; no change in fatal plus injury crashes 9% increase in total crashes; no change in fatal plus injury crashes 44th street interchange; auxiliary lane between both EN-EX combinations No change in total crashes; 14% reduction in fatal plus injury crashes STEP 4—Signing: The I-121 corridor has many interchanges spaced one mile apart due to the design of the arterial network. This interchange spacing results in exit ramps being spaced approximately one mile apart as well. If one-mile advance guide signs were used, they would be located in the vicinity of the gore of the upstream exit, which is discouraged by the MUTCD. Instead, the state has chosen to place many advance guide signs in this corridor at 1 ¼ or ¾ of a mile prior to an exit. Interchange sequence signs are used as well. When a ¾- mile advance guide sign is used, it is generally followed by a ¼-mile advance guide sign instead of a ½-mile advance guide sign. In addition to spreading the signs, this places the second advance guide sign beyond the end of the Applying Guidelines Equation 5-1 to Segment 2, the total expected number of crashes on the northbound freeway mainline between 44th street and 48th street is 13 crashes per year without an auxiliary lane, 11 crashes per year with an auxiliary lane. Approximately 4 crashes per year are expected to be fatal plus injury on the northbound freeway mainline between 44th street and 48th street without an auxiliary lane, approximately 3 crashes per year with an auxiliary lane. Appendix A 125

upstream entry ramp. This makes the sign visible to drivers entering the freeway on the entry ramp, and in some cases it makes the sign assembly easier to construct by moving it away from the gore. These same principles have been applied to the segment of the I-121 corridor near the 44th Street interchange, and the signing plan shown in Figure 3-8 was developed. The signing plan includes signs for SR 63 (52nd Street), which is 1 mile north of 48th Street, as well as signs for 32nd and 28th Streets, which are 2 and 3 miles south of 40th Street, respectively. All signs are overhead because of the number of lanes on the freeway and the potential for congestion. The signing plan was developed assuming auxiliary lanes between ramps and single-lane exit ramps. Double-lane exit ramps that maintain lane balance will be more complex to sign as they will need to indicate that two lanes go to an exit. If the double-lane exit ramp option is further considered, a signing plan should be developed for it. However, the signing plan for the single-lane exit ramp option indicates that the basic concept of an interchange at 44 Street is feasible from a signing perspective. Other Considerations: The proposed interchange will be on the Interstate Highway System and therefore must be approved by FHWA, which considers eight points before granting or denying access. The state has already entered into discussions with FHWA regarding the 44th Street interchange, and FHWA has indicated they will approve the interchange if the eight points are satisfied. FINDINGS Based on the conceptual level of ramp spacing analysis conducted, plans for the 44th Street interchange do not have any fatal flaws. All four of the new ramps will be close enough to existing ramps that auxiliary lanes are recommended and weaving sections will be created. A complete HCM analysis for these segments should be conducted to see if they will be below capacity and to see if they will meet the state’s operating guideline of LOS E. No major decrease in freeway mainline safety is expected if auxiliary lanes are provided between all ramp pairs. No increase in crashes on the freeway mainline is expected if auxiliary lanes are provided between all EN-EX ramp combinations. All spacing considerations should be reevaluated during the project’s preliminary design phase. 126 Guidelines for Ramp and Interchange Spacing

Appendix A 127

Case Study 4 Case Study 4 illustrates the modernization of a 1950’s vintage freeway. The study is being conducted because of basic capacity constraints on the highway mainline, and also to address traffic operational and safety conditions that result from relatively short ramps at the Stone Road/Plant Drive interchange. The basis of the previous three case studies has been the addition of access to a freeway. In this case study, existing accesses are evaluated in light of capacity and safety concerns. The range of possible solutions would include assessing whether the highway interchanges should be maintained and, if so, how they should be modified to address documented operations and safety conditions while serving forecast traffic. BACKGROUND General A state transportation agency is rebuilding a highway (SR 53) that was constructed in the 1950s with a design speed of 50 mph. The agency has identified operational and safety deficiencies associated with the Stone Road/Plant Drive and SR 71 interchanges. The Stone Road/Plant Drive interchange has short, low-speed hook ramps, and the state agency is investigating replacement options including the following: • Removing highway access at this location; • Reconstructing the hook ramps to provide a contemporary diamond interchange; • Providing a diamond interchange with C-D roadway system to SR 71; and, • Providing a diamond interchange with braided ramps to SR 71. The state and the local business community would prefer to maintain access, and do so with a contemporary diamond form as it will be less expensive than braided ramps or C-D roads. Partial cloverleaf interchange forms are considered infeasible due to the number of properties that would need to be acquired. The C-D or braided-ramp concepts would be considered in detail only if sufficient ramp spacing cannot be achieved without them. The SR 71 interchange has low-speed curves on its ramps, and it does not provide adequate acceleration and deceleration length along the freeway. Segment speeds are also negatively influenced by a lack of acceleration and deceleration at the ramps at SR 53 to the east. Based on prior studies, the SR 71/SR 53 interchange will likely be reconstructed to a partial cloverleaf form to remove weaving associated with the consecutive loops serving eastbound SR 53 and northbound SR 71 traffic. The Stone Road/Plant Drive interchange will be reconfigured to a diamond form and the SR 71 interchange ramps will be reconfigured to a partial cloverleaf form. The focus of the ramp spacing assessment will be understanding the operational and safety relationships between these reconfigured interchanges. 128 Guidelines for Ramp and Interchange Spacing

Adjacent Interchanges The Stone Road/Plant Drive interchange and the SR 71 interchange are separated by 3,600 ft, measured from the centerline of SR 71 to the approximate center of the Stone Road ramp area (Stone Road does not cross SR 53 at the interchange). These two interchanges are several miles away from any other interchanges. The project area is shown in Figure 4-1. Traffic Volumes and Characteristics SR 53 has three lanes in each direction through the project area, and carries 3,700-4,500 vehicles per hour on the segment between the two interchanges during the peak hour. Ramp volumes at the SR 71 interchange are higher than those at the Stone Road/Plant Drive interchange. Heavy vehicles account for 5% of the volume on the freeway, and terrain in the area is level. While heavy-vehicle traffic is moderate during the peak periods, adjacent aggregate mining results in a consistent stream of trucks on a 24-hour basis. This effect of these heavy vehicles further degrades traffic operations because of the inadequate Stone Road/Plant Drive ramps. Traffic volumes and spacing dimensions are shown in Figure 4-2. AGENCY REQUIREMENTS The state in which this project is located has an operating guideline of LOS D for urban freeways. Additionally, the state has a minimum interchange spacing guideline of one-mile urban areas. This spacing guideline was adopted many years after the SR 53 highway was constructed. The state has indicated they will accept spacing shorter than the one-mile guideline for this project if both of the following criteria are met: • Existing access to adjacent land uses can be preserved (i.e., there is an existing interchange in place, even though it may be entirely rebuilt as part of this project). • A traffic study demonstrates there will be no adverse impacts to SR 53 traffic operations. RAMP SPACING CONSIDERATIONS The following ramp spacing assessment follows the sequence outlined in Section 5.3: • Geometry considerations, • Traffic operations, • Safety, and • Signing. Appendix A 129

F IGURE N CAS E S T UDY 4 S ITE MA P – EXISTIN G 4- 1 S T O N E R O A D 71 53 P L AN T DRI V E NOR THE AS T R IVE R Source: Digital Globe

FIGURE N CASE STUDY 4 EXISTING RAMP SPACING AND DAILY TRAFFIC VOLUMES 4-2 Note: Spacing defined from approximate location of merging and diverging painted tip 800 100 0 SR 71 50 0 500 Plant Drive Stone Road Stone Road 500’ 1000’2800’ spacing 500’ 1600’2200’ spacing 800 70 0 600 ∼4500 (1500/lane) 400 30 0300 ∼3700 ∼4500 ∼2900 ∼4100 4300’ ∼3700 (1233/lane)

STEP 1—Geometric considerations: The first step is to conceptually determine the form of the rebuilt interchanges. For Stone Road/Plant Drive, a diamond form will be considered initially. This will minimize cost and right-of-way impacts and create a full interchange (currently there is no ramp from Stone Road to SR 53 westbound). As part of the interchange reconstruction, the Stone Road underpass will be removed, and Plant Drive will be realigned to pass under SR 53 between the entry and exit ramps. Improvements will be made to the SR 71 interchange as well. A conceptual plan for the proposed interchanges is shown in Figure 4-3. As shown in Figure 4-4, options for the Stone Road/Plant Drive interchange to be reconstructed near its existing location will require collector-distributor roads or braided ramps. These will require at least two additional structures across the Northeast River. This will greatly increase the project’s cost and environmental impact, potentially delaying improvements to the SR 53 corridor for many years. In addition, community outreach has resulted in stakeholder preferences to minimize construction in and around the river. Using a typical diamond form but placing the interchange further to the east will impact a number of properties and buildings, which the state also wishes to avoid if possible. Stone Road Interchang e The eastern ramps of the Stone Road/Plant Drive interchange (SR 53 westbound offramp and SR 53 eastbound onramp) were initially constructed to avoid widening the SR 53 bridge over the Northeast River. At the time the highway was built in the early 1950s, the area was virtually undeveloped and traffic volumes where extremely low. A contemporary, reconstructed interchange at this location will result in ramps that extend to the bridge over the Northeast River. At a conceptual level and starting point in laying out diamond interchange ramps, physical entry and exit ramp gores are approximately 1,000 ft from the crossroad. This distance generally meets vertical alignment needs for making appropriate grade changes and incorporating desired ramp geometry. The distance from a painted diverging tip to the crossroad generally ranges from 1,300-1,500 ft, and the distance from the crossroad to the painted merging tip generally ranges from 1,400-1,800 ft. This is based on an entrance ramp having a distance between the physical gore and the painted merging tip typically in the range of 400-800 ft, based upon the horizontal curvature of the ramp and whether a taper or parallel entrance is used. On an exit ramp, the distance from the painted diverging tip to the physical gore typically varies from 300-500 ft for similar reasons. A number of options exist for the rebuilding of the Stone Road/Plant Drive interchange. For cost, impact, and community input reasons, the concept shown in Figure 4-3 is preferable and is the focus of the analysis presented here. However, other options should be considered until a more detailed analysis is able to determine if the concept in Figure 4-3 is feasible from the perspective of geometry, traffic operations, signing, and safety . 132 Guidelines for Ramp and Interchange Spacing

F IGURE N CAS E S T UDY 4 S ITE MA P – PROPOSED 4- 3 S TO N E RO A D 71 53 P L AN T DRI V E NOR THE AS T R IVE R Source: Digital Globe

Source: Digital Globe 134 Guidelines for Ramp and Interchange Spacing

Based on the ramp dimensions noted previously (1,300-1,500 ft and 1,400- 1,800 ft), and the 3,500-foot distance between the exiting Stone Road/Plant Drive interchange location, the reconstructed Stone Road interchange will need to be shifted to the west to maximize ramp spacing dimensions to SR 71. In addition, ramp lengths will need to be minimized in such a way as to serve forecast volumes while maximizing the ramp spacing dimensions. Attaining a design that optimizes the ramp geometry and maximizes ramp spacing values appears potentially feasible given the relatively low ramp volumes and estimated queues at the ramp-terminal intersections. Based upon the ramp lengths, it appears infeasible to locate the reconstructed Stone Road interchange in such a way as to avoid ramp entrance and exit ramp tapers prior to the Northeast River bridges. Therefore, these mainline bridges will need to be widened or reconstructed to accommodate the exit and entry gore areas. While not desirable based on community input, this configuration would provide a reduced footprint and area of impact in this sensitive area compared to the C-D and braided-ramp concepts. SR 71 interchange A partial cloverleaf “A” form will be provided at the SR 71 interchange, based on prior studies. The eastbound-to-northbound loop ramp will be eliminated and the remaining ramps reconfigured to serve all movements to and from SR 53 eastbound. This would eliminate the weaving sections on SR 53 eastbound and SR 71 northbound between the loop ramps. The existing diagonal ramps will be modified to remove low-speed curves to the extent feasible within the available right-of-way. Resultant Spacing The changes to the Stone Road/Plant Drive and SR 71 interchanges noted above will result in the following ramp spacing dimensions: • Approximately 1,900 ft between the Stone Road/Plant Drive onramp and the SR 71 offramp on SR 53 eastbound. • Approximately 1,600 ft between the SR 71 southbound onramp and the Stone Road/Plant Drive offramp on SR 53 westbound. These spacings, as well as traffic volumes, are shown in Figure 4-5. STEP 2—Traffic Operations: The ramp spacing on SR 53 between the two rebuilt interchanges will be between 1,500-2,000 ft. AASHTO policy recommends the consideration of an auxiliary lane when the spacing between an entry ramp and an exit ramp is Lengthening the Stone Road/Plant Drive ramps to provide adequate acceleration and deceleration lengths shortens the spacing to the SR 71 ramps. In the westbound direction, the approximate ramp spacing dimension will be at the recommended AASHTO minimum (per Exhibit 10-68) of 1,600 feet for an entry-exit combination on a full freeway. In the eastbound direction, the approximate ramp spacing dimension will be 300 feet greater than the recommended AASHTO minimum. A detailed traffic operations analysis should be conducted to determine if these spacings are feasible, and preliminary design will later determine if the approximate dimensions used in this initial analysis are appropriate. Appendix A 135

FIGURE N CASE STUDY 4 PROPOSED RAMP SPACING AND TRAFFIC VOLUMES 4-5 Note: Spacing defined from approximate location of merging and diverging painted tip 800 100 0 SR 71 50 0 1200 Plant Drive Stone Road 1400’ 1000’1900’ spacing 1300’ 1400’1600’ spacing 800 600 ∼3700 (1233/lane) ∼4500 (1500/lane) 400 300300 ∼4500 ∼3700 ∼2900 ∼4100 4300’ 400

1,500 ft or less. Ramp spacing dimensions here are not greatly in excess of 1,500 ft and an auxiliary lane could be used. However, adding a full lane on both of the Northeast River bridges could greatly increase the cost of this project, so auxiliary lanes will only be used if they are needed to fulfill the state’s LOS guideline. If the river bridges must be completely reconstructed because of their condition or inability to be modified to serve the entrance and exit ramp tapers, the auxiliary lane should be included. If auxiliary lanes are to be used, designers should consider providing lane balance at the reconstructed exit ramp terminals to reduce the effects of weaving. To consider worst-case conditions, this initial operational analysis should be conducted under the assumption that no auxiliary lane is present. If an auxiliary lane is added to the design, operations will be improved. Since this is a three-lane freeway, the planning-level HCM merge and diverge analysis charts developed by the project team and discussed in Chapter 4 of the Guidelines may be used. Eastbound—Analysis Dashed lines on Figure 4-6 indicate the proposed ramp spacing of 1,900 ft (horizontal line) and the freeway volume of 4,500 vph (vertical line). In order for acceptable operation (LOS D or better) to occur, the blue lines must intersect above and to the left of the curve that corresponds with the ramp volumes that are present. On SR 53 eastbound, the entrance ramp volume is 300 vph. A set of entrance ramp volume curves for 500 vph exits on the chart, and these may be used to conduct a more conservative analysis (a user of the chart could also extrapolate a set of curves for 300 vph entrance ramps). The exit ramp volume on SR 53 eastbound is 1,200 vph, so the 1,200 vph exit ramp curve (dashed curve) within the 500 vph entrance ramp set of curves should be used as the LOS D threshold in this case. Appendix A 137

Figure 4-6 Operational Evaluation of SR 53 Eastbound Between Stone Road and SR 71 Eastbound—Findings Under the expected conditions on SR 53 eastbound, Figure 4-6 indicates that LOS D will be achieved if the volume on the entrance ramp is 500 vph and the volume on the exit ramp is not much over 1,200 vph. These volume thresholds are slightly higher than the volumes projected for the Stone Road/Plant Drive entry ramp (300 vph) and the SR 71 exit ramp (1,200 vph). This indicates that the proposed ramp spacing may meet the state’s LOS guideline. A complete HCM analysis should be conducted to determine this with certainty because this specific case is very near the chart’s threshold and a number of assumptions have been built into the chart (vehicle mix, peak- hour factor, etc.) Westbound A similar analysis, conducted for the westbound direction, is shown in Figure 4-7. In this case, a set of entrance ramp curves was interpolated because the actual entry-ramp volume (1,000 vph) was not depicted on the chart. The 138 Guidelines for Ramp and Interchange Spacing

actual exit-ramp volume (400 vph) is not shown as well, so the 800 vph was used instead and will result in a more conservative analysis. A portion of the interpolated curve for a 1,000-vph entry ramp and 800-vph exit ramp is shown on the chart. Figure 4-7 indicates that the proposed spacing should result in acceptable operation on SR 53 westbound. Figure 4-7 Operational Ev aluation of SR 53 Westbound Be tw een Stone Road and SR 71 STEP 3—Safety: The focus of the safety analysis for this case study is the entrance-exit ramp combinations between Plant Drive/Stone Road and SR 71 to determine if a conventional diamond interchange without C-D roads or braided ramps is feasible at Stone Road/Plant Drive. T he safety assessment should consider tradeoffs between increasing speed change lane lengths while reducing ramp spacing. The simplified HCM analysis in Figures 4-6 and 4-7 indicates that the interchange concept shown in Figure 4-3 may be feasible and should be further investigated. It may be possible to construct a diamond interchange on SR 53 at Stone Road/Plant Drive w ithout using braided ramps or collector-distributor roadways or moving the interchange further to the east. Appendix A 139

Ramp spacing values currently are 2,200 (westbound) and 2,800 ft (eastbound). Guidelines Exhibit 5-5 (reproduced below as Figure 4-8) indicates relative crash risks for these spacing values of -7% and -11%, respectively. Reconstructing the Stone Road/Plant Drive interchange to a conventional diamond and the SR 71 interchange to a partial cloverleaf form will result in projected ramp spacing dimensions of 1,600 ft (westbound) and 1,900 ft (eastbound). Relative crash risks for these new spacing values are zero and -4%, respectively. A significant safety improvement is expected by removing the mainline weaving section at the SR 71 interchange. Research to make a reliable estimate of this safety improvement does not exist, but eliminating the loop ramp greatly reduces the number of vehicle conflicts. 1 Relative crash risk is measured by the percent difference in crashes, of all types and severities, at some ramp spacing value compared to a ramp spacing of 1,600 ft Figure 4-8 Preliminary Safety Assessment Tool for Ramp Spacing, Entrance Ramp Followed by Exit Ramp (Guidelines Exhibit 5-5) The AASHTO Highway Safety Manual includes quantitative safety information associated with lengths of speed change lanes. The manual suggests that for acceleration and deceleration lane lengths less than 690 ft (the condition that existed prior to the Stone Road/Plant Drive interchange improvement), an 11% reduction and a 7% reduction in crashes of all types The expected number of crashes on the freeway mainline between Stone Road/Plant Drive and SR 71 is expected to increase by around 7% as a result of a reduction in spacing. This estimate assumes no safety gain as a result of the reconstruction of the Stone Road/Plant Drive interchange and the SR 71 interchange, which is unrealistic. Overall, a net improvement in safety is likely by reconstructing the Stone Road/Plant Drive interchange to a conventional diamond and the SR 71 interchange to a partial cloverleaf. Quantitative estimates of the total net improvement cannot be made due to existing gaps in related safety research. and severities is expected for every 100 ft increase in the acceleration lane and 140 Guidelines for Ramp and Interchange Spacing

Additional safety gains (approximately 20% reduction in expected crashes along the mainline) are possible by providing auxiliary lanes between the Stone Road/Plant Drive and SR 71 interchanges (see Guidelines Section 4.5.4.1.4). The benefit should be compared to costs of a wider bridge across Northeast River to accommodate the extra lane. The expected 7% increase in mainline crashes resulting from the spacing reduction assumes the spacing acts in isolation. Models used to create Exhibit 5-4 and make this estimate used data from interchanges with more conventional acceleration and deceleration lanes than the current Stone Road/Plant Drive interchange. STEP 4—Signing: On SR 53 westbound, there is presently one location upstream of the SR 71 interchange with two sign panels – a ½ mile advance guide sign for SR 71 and a 1 ½ mile advance guide sign for Plant Drive and Stone Road. This provides adequate advance notice of the Plant Drive/Stone Road exit. In the westbound direction, there is no other location with more than one sign panel. On SR 53 eastbound, signing will be simplified by removing the loop ramp to SR 71 northbound. Currently there are two sign panels at the gore for the SR 71 southbound ramp and a ½ mile upstream of the gore. When the loop ramp is removed and replaced with a single-exit design, only one sign panel will be needed at each of the locations that currently have two. This sign panel will also have one less message unit than each of the existing panels because it will not be necessary to indicate which direction of SR 71 the ramp will serve (it will serve both directions). A ½ mile before the Plant Drive/Stone Road exit ramp, there is one additional location with two sign panels (one advance guide sign for Plant Drive/Stone Road and one advance guide sign for SR 71). This is not problematic and can remain after improvements are implemented. No additional exit ramps are being added on SR 53 as part of this project. In the westbound direction, the number of ramps will remain the same. In the eastbound direction, one of the two exit ramps at SR 71 will be eliminated. All exits currently have at least two advance guide signs which are placed in a manner that adheres to the current MUTCD. At no location are there more than two sign panels. No signing issues are anticipated. FINDINGS At the first stage of conceptual development, the proposed rebuilding of the Stone Road/Plant Drive and SR 71 interchanges shown in Figure 4-3 appears feasible from a ramp and interchange spacing perspective. Based on forecast ramp and freeway volumes and anticipated ramp spacing dimensions, it appears that the state’s LOS D guideline will be satisfied. No ramps are added, and signing needs can easily be accommodated in a manner that is consistent with the MUTCD. The safety analysis suggests an overall No additional ramps are being added. Each direction of the highway will have only two exits, so there will not be a need for more than two sign panels at any location. The simplified analysis conducted thus far indicates that it may be possible to achieve adequate ramp spacing between the SR 71 interchange and the Plant Drive/Stone Road interchange without using collector-distributor roadways or braided ramps. deceleration lane, respectively. Additional, incremental safety improvements are not expected once the speed change lane reaches 690 ft or longer. Appendix A 141

net improvement in safety following the reconstruction of the Stone Road/Plant Drive interchange to a conventional diamond and the SR 71 interchange to a partial cloverleaf. These conclusions are highly dependent upon the assumed ramp lengths and should be reevaluated as the design is further developed. If ramps need to be lengthened, auxiliary lanes or alternate interchange forms may be needed. 142 Guidelines for Ramp and Interchange Spacing

Case Study 5 Case Study 5 illustrates ramp spacing considerations for adding new freeway connections in a complex environment where many ramps already exist. This case study evaluates a ramp braid and other relatively complex ramp solutions and highlights the role that signing plays in ramp spacing considerations. BACKGROUND General A circumferential interstate (I-233) passes through a heavily developed urban area with several complex interchanges. This portion of the I-233 loop is signed as an east-west route. The state transportation agency is proposing new connections to I-233 to improve access to Foothills Drive and enhance area circulation. The design of these new connections is complicated by existing interchange ramps that are in close proximity to Foothills Drive. These existing ramps serve an existing service interchange to Executive Drive and a connection to the international airport access road. The ramp configuration in the eastbound direction of I-233 was recently established and is not included in this current evaluation. Similarly, the on-ramp from Foothills Drive to I-233 eastbound will be a diagonal ramp that, although it will be close to the Sunset Street offramp, will not create any operational issues. Therefore, the focus of this exercise is to establish the configuration of the westbound exit ramp to Foothills Drive while considering the existing westbound exit ramps at Executive Drive and the airport access road. The planning considerations of the westbound ramp configuration, with a focus on spacing considerations related to the Foothills Drive exit ramp, are presented in the following sections. Adjacent Interchanges The footprint of the partial, directional interchange between I-233 and the airport access road includes the Foothills Drive overpass at I-233. Other than this, the nearest interchange to the west is a single-point diamond at Sunset Street that is 6,200 ft away from Foothills Drive (centerline-to-centerline of each crossroad). To the east, the nearest interchange is a diamond at Executive Drive that is 3,500 ft away from Foothills Drive (centerline-to- centerline). These roads and interchanges are shown on the site map in Figure 5-1. Traffic Volumes and Characteristics Figure 5-2 depicts traffic volumes and existing interchange and ramp spacing along I-233. In addition, the figure schematically shows the approximate configuration of traditional diagonal ramps. The number of westbound basic lanes on the interstate decreases through this area. There are four basic lanes upstream of the diverge to the airport and three basic lanes downstream. As Appendix A 143

F IGURE N CASE STUDY 5 SITE MAP 5- 1 Internationa l Airpor t Airpor t Access Roa d Su n se t St re et F o o t h i l l s D r i v e t 233 233 FLOODPLAI N L AKE 67 Proposed New Interchange

FIGURE N CASE STUDY 5 EXISTING RAMP SPACING, NEW MOVEMENTS TO BE SERVED, AND TRAFFIC VOLUMES 5-2 Executive Drive Foothills Drive Interchange Spacing Inte rch a ng e Sp a cing Sunset Street 1900 ’ 1900’ 4900 500 600 300 200 2700 7000 1100 5400 6200 ’ 6200’ 1500 ’ sp acing 2800 ’ Airport Access 1,800' available— insufficient for ramp 1900’ 3500’ NO SPACING BETWEEN POTENTIAL RAMPS — ALTERNATE DESIGN NEEDED 7600 7300 7500 1700’ Notes: 1. Spacing defined from approximate location of merging and diverging painted tip 2. Movement to be served between I-233 and Foothills Drive shown in black

Figure 5-2 shows, there is insufficient ramp spacing for conventional diagonal ramps between Foothills Drive and the existing interchange ramps at Executive Drive. Presently, there are only 1,800 ft of spacing between the merging tip of the Executive Drive onramp and the centerline of Foothills Drive. If the diverging tip of a ramp to Foothills Drive were to be placed at the same location as the merging tip of the ramp from Executive Parkway (which in itself is not a feasible design), the resulting exit ramp would still be shorter than the other exit ramps in the corridor. Westbound volumes are highest during the p.m. peak period, with heavy vehicles accounting for approximately 10% of the volume on the interstate and less than 5% of the volume on the arterials. Terrain in the area is rolling. Due to the proximity to the airport and a high percentage of tourists, a significant number of drivers on I-233 will be unfamiliar with the area. Since there is no room for typical diagonal exit ramps at Foothills Drive, alternate ramp and interchange forms will be required. These are discussed in the Ramp Spacing Considerations section. AGENCY REQUIREMENTS The state in which this project is located has an operating guideline of LOS D for urban interstates. The new interchange should not result in any components of the freeway operating below LOS D. ALTERNATIVES UNDER CONSIDERATION Due to the complexity of existing ramps in this area, the most desirable form of ramps for a connection to Foothills Drive is not immediately clear. Three alternatives have been proposed and are depicted schematically in Figures 5-3 A, B, and C: • Braided ramps—The Executive Drive entry ramp and Foothills Drive exit ramp would be braided (grade separated). • Double exit with frontage road—There would be two separate exits from I-233 to Executive Drive and Foothills Drive. The existing westbound onramp from Executive Drive would be removed, and this traffic would connect to the frontage road and pass through the ramp-terminal intersection at Foothills Drive • Single exit with frontage road—There would be a single exit from I- 233 for Executive Drive and Foothills Drive, and the connection to Executive Drive would depart the single ramp as a “turning roadway.” The existing westbound entrance ramp from Executive Drive to I-233 would be removed, and this traffic would connect to the frontage road and pass through the ramp-terminal intersection at Foothills Drive. This section provides an overview of other alternatives that might be investigated beyond traditional diamond ramps. 146 Guidelines for Ramp and Interchange Spacing

CASE STUDY 5 — PROPOSED RAMP SPACINGS AND TRAFFIC VOLUMES (BRAIDED RAMPS ALTERNATIVE) FIGURE 5-3A 7,300600 Notes: 1. Spacing defined from approximate location of merging and diverging painted tip 2. New ramps shown in black (NO SCALE) N Interchange Spacing 200 1,300' – 1,900' 7000 1,400' – 1,600 2,000' – 2,200

FIGURE N CASE STUDY 5 PROPOSED RAMP SPACING, AND TRAFFIC VOLUMES (DOUBLE EXIT ALTERNATIVE) 5-3B 7300600 Interchange Spacing 200 3,800' – 4,200' 1,000' – 1,400' Fo ot hi lls D ri ve Ex ec ut iv e D ri ve Airport Access Notes: 1. Spacing defined from approximate location of merging and diverging painted tip 2. Movement to be served between I-233 and Foothills Drive shown in black

FIGURE N CASE STUDY 5 PROPOSED RAMP SPACING, AND TRAFFIC VOLUMES (SINGLE EXIT ALTERNATIVE) 5-3C 7300600 Interchange Spacing 200 5,200' 6700 900 800 600 Fo ot hi lls D ri ve Ex ec ut iv e D ri ve Airport Access Notes: 1. Spacing defined from approximate location of merging and diverging painted tip 2. Movement to be served between I-233 and Foothills Drive shown in black

RAMP SPACING CONSIDERATIONS A planning-level analysis will now be conducted for each of the alternatives to determine if they are viable from a ramp spacing perspective. The following topics that influence ramp spacing will be considered: • Geometric considerations, • Traffic operations, • Safety, • Signing, and • Other considerations. STEP 1—Geometric considerations: Braided Ramps (See Figure 5-3A) The design of braided ramps for these interchanges should attempt to keep the painted tip of the reconstructed Executive Drive onramp at its current location to avoid reducing the ramp spacing dimension to the exit to the airport access road. This requires locating the diverging tip of the new Foothills Drive offramp as far upstream as necessary to achieve vertical clearances between ramps. The ramps would be braided with the Executive Drive onramp passing over the new Foothills Drive offramp since the Executive Drive onramp is already elevated. The distance required to achieve adequate grade separation between the two ramps will influence the location of the diverging tip of the new Foothills Drive offramp. The first step is to determine the approximate location of the ramp crossing location. Given that Executive Drive passes over I-233, the existing ramp-terminal intersection is already approximately one level above the interstate. The crossing angle of the two roadways should not be too flat in order to avoid complex bridge designs (extra long bridges or straddle bent supports). It also should be located as near the ramp terminal intersection as possible so that the reconstructed Executive Drive onramp can reach grade on I-233 without appreciably shortening the ramp spacing dimension to the airport access Road exit. Therefore, the crossing location might be targeted 200-300 ft downstream of the ramp-terminal intersection. The new Foothills Drive exit ramp should be located so that it is not hidden by the Executive Drive overcrossing. Ideally, the physical gore would be located 100-200 ft in advance of the overcrossing for maximum visibility by drivers on westbound I-233. This is sometimes unattainable depending on the lateral clearance of the existing overcrossing bridge abutment, and adjustments must sometimes be made to avoid reconstructing existing overpasses. This places the painted tip of the exit ramp approximately 300- 500 ft in advance of the Executive Drive overcrossing and approximately 2,000-2,200 ft from the reconstructed Executive Drive westbound onramp. When considering this alternative, the first step is to locate the new ramp braid while attempting to maintain or minimally impact existing ramp spacing between the ramps at Executive Drive and the airport access road. The second step is to consider the three- dimensional roadway design needs to attain the grade separation between the new Foothills Drive ramp and the reconstructed Executive Drive ramp. 150 Guidelines for Ramp and Interchange Spacing

This provides approximately 1,400-1,600 ft to the upstream exit to Executive Drive. As designers advance the ramp-braid concept, the focus will be on optimizing the spacing between the proposed exit ramp and the up- and downstream ramps to and from Executive Drive. Designers must balance the tradeoffs in ideally locating the new exit ramp, attaining adequate vertical and horizontal alignments for the new and reconstructed ramps, and maximizing ramp spacing values between the series of entrance and exit ramps along this segment of I-233. Double Exit with Frontage Road (Figure 5-3B) This alternative would eliminate the Executive Drive onramp connection to westbound I-233 and combine it with a new exit ramp for Foothills Drive. Many issues will influence where the diverging tip for the Foothills Drive offramp should be placed. These include the following: • The exit to the airport is a major fork with a two-lane exit ramp and a reduction in the number of basic lanes on the freeway. It would be desirable to place the new exit ramp as far upstream (and away from the airport exit) as possible. However, at this project location, this upstream distance must be balanced by considering the location of the existing Executive Drive overcrossing. • The new exit should be placed as far as reasonably possible from the existing westbound I-233 exit to Executive Drive. This places the proposed exit in the vicinity of the Executive Drive overcrossing. Exits immediately beyond or directly under an overpass are undesirable because they frequently can not be seen. Placing the diverging tip for the proposed Foothills Drive exit prior to the Executive Drive overpass would eliminate this issue, although it would bring the Foothills Drive exit ramp and the Executive Drive exit ramp closer together. • The westbound traffic from Executive Drive would now use the frontage road and pass through the ramp-terminal intersection at Foothills Drive. The Foothills Drive exit should be placed far enough upstream to accommodate this merge on the frontage road, and provide a section of the frontage road downstream of the merge long enough to accommodate lane changing and queuing associated with the ramp-terminal intersection. • The merge location on the frontage road will be influenced by the need to bring the new ramp and the Executive Drive connection to the same grade. The current Executive Drive ramp-terminal intersection is approximately one level above I-233. The proposed ramp will be raised to meet a falling reconstructed connection The number of ramps associated with this alternative will be difficult to accommodate and will result in minimal spacing. Appendix A 151

between Executive Drive and the new frontage road. The frontage road would then connect to Foothills Drive. Considering these issues, the most feasible location for the ramp-freeway junction of the new Foothills Drive exit ramp appears to be between the existing Executive Drive exit ramp and the Executive Drive overpass. The 2004 AASHTO Policy (Exhibit 10-68) recommends a 1,000-foot minimum spacing between successive exit ramps. Using this dimension as a starting point, the diverging tip of the proposed Foothills Drive exit ramp would be located approximately 900 ft upstream of the Executive Drive overpass, and the physical gore would be nearly underneath the overpass, an undesirable condition. This design could be improved by moving the proposed Foothills Drive exit ramp several hundred feet upstream and locating the gore and exit ramp taper further in advance of the Executive Drive overpass. This design would create a spacing of less than 1,000 ft between successive exit ramps from westbound I-233 to Executive Drive and the proposed Foothills Drive ramp. The existing Executive Drive exit gore could be shifted upstream to create a 1,000-foot spacing. Modifications to the Executive Drive exit gore will be difficult and expensive because I-233 is on an elevated section upstream of the gore. The more feasible design option may be to not modify the Executive Drive exit gore and create an entry-entry ramp spacing of less than 1,000 ft. The 1,000-foot dimension is recommended but not a standard, and in complex environments such as this location, it will not always be possible to achieve recommended spacing values. Single Exit with Frontage Road (Figure 5-3C) This alternative would also replace the existing Executive Drive entry ramp, combining this movement to the new frontage road, and allowing traffic to pass through the ramp-terminal intersection at Foothills Drive. The current Executive Drive exit from I-233 would be modified to a single-exit configuration that serves Executive Drive and Foothills Drive, with the Executive Drive movement diverging from the single exit as a turning roadway. This configuration, like the previous configuration, eliminates the weaving section on I-233 prior to the airport exit. STEP 2—Traffic Operations: The three alternatives described above will result in the ramp spacings that are listed below in Tables 5-1 through 5-3 and shown in Figures 5-3A through 5-3C. The double-exit design eliminates the weaving section prior to the airport exit, but like the braided ramp alternative, it places the diverge for the Foothills Drive ramp near the Executive Drive overpass and close to the Executive Drive exit ramp. Executive Drive traffic must pass through the Foothills Drive ramp-terminal intersection. This complex environment reinforces the need to more closely evaluate traffic operations analyses and consider design adjustments as the evaluations move forward. By having fewer ramps, the single-exit design increases the spacing of remaining ramps compared to the other scenarios. However, this configuration concentrates traffic to the single-exit and requires Executive Drive traffic to pass through the Foothills Drive ramp-terminal intersection. 152 Guidelines for Ramp and Interchange Spacing

Table 5-1 Ramps in project area on I-233 Westbound—braided ramps alternative. Ramp Resultant Spacing Offramp to Executive Drive Exit-exit with 1,400 ft to 1,600 ft spacing Offramp (braided) to Foothills Drive Exit-entry with 2,000 ft to 2,200 ft spacing Onramp (braided) from Executive Drive, with lane from ramp continuing as auxiliary lane Entry-exit weaving section with 1,300 ft to 1,900 ft spacing Offramp (double lane) to the airport. One auxiliary lane and one freeway lane dropped Table 5-2 Ramps in project area on I-233 Westbound – double exit with frontage road alternative. Ramp Resultant Spacing Offramp to Executive Drive Exit-exit with 1,000 ft to 1,400 ft spacing Offramp to Foothills Drive Exit-exit with 3,800 ft to 4,200 ft spacing Offramp (double lane) to the airport. One auxiliary lane and one freeway lane dropped Table 5-3 Ramps in project area on I-233 Westbound – single exit with frontage road alternative. Ramp Resultant Spacing Offramp to Executive Drive Exit-exit with 5,200 ft spacing Offramp (double lane) to the airport. One auxiliary lane and one freeway lane dropped The three alternatives, with ramp spacings detailed in Tables 5-1 to 5-3, will result in one or more closely spaced ramp combinations. In the next phase of ramp sequencing investigations, a complete operational analysis of each alternative still under consideration should be performed. The HCM procedures are best suited for analyzing individual ramp-highway junctions and weaving sections. Complex environments may benefit from applying other analysis tools. Simulation models may need to be employed to address the complex interrelationships of the ramp configurations. At this planning stage, the qualitative traffic operations analyses can be used to compare each alternative ramp combination. In addition to operation of the freeway, operation of individual ramp-terminal intersections should also be considered when comparing the alternatives. Complex configurations may sometimes preclude applying planning- level operations tools. Complex configurations, such as this, require a special emphasis on traffic operations at the earliest stage of the project’s development. Appendix A 153

Figure 5.4 provides schematic diagrams of the lane numbers and arrangements for each of the alternative concepts. Traffic operational considerations for each of the concepts are described in the following sections: Braided Ramps The braided-ramp alternative creates the greatest number of ramps on I-233. The first ramp combination encountered by drivers on I-233 will be the offramp to Executive Drive and the offramp to Foothills Drive. Although exit-exit ramp combinations generally have a minimal impact on traffic operations, the close spacing dimension may result in unacceptable traffic flow. The weaving section between the Executive Drive entrance and the airport exit is of greater concern. This section already exists and experiences poor operation. Traffic entering the freeway from Executive Drive must make two lane changes to remain on I-233 instead of exiting to the airport. This may be a fatal flaw for this configuration. This maneuver will be difficult when volume on the airport exit is high. The freeway will have five lanes upstream of the major fork to the airport access road, and downstream of the fork there will be five lanes as well (three on I-233 and two on the airport access road ramp). Such a design will violate the principles of lane balance as discussed in Chapter 3 of the Guidelines. This design is undesirable from a traffic operations perspective and because it retains the weaving section that currently experiences poor operation. Double Exit with Frontage Road By introducing a frontage road, this alternative eliminates the Executive Drive onramp and the resulting weaving section between it and the airport exit. From this perspective, this alternative is superior to the braided ramp alternative in terms of traffic operations. This alternative provides lane balance. Other ramp spacing values are similar to the braided-ramp alternative. Single Exit with Frontage Road Compared to the other alternatives, this alternative eliminates the weaving section prior to the airport exit. In addition, it removes the design challenges of locating the new Foothills Drive exit ramp in the vicinity of the Executive Drive overpass and near the Executive Drive exit. The new combined Executive Drive and Foothills Drive exit would have a higher volume than the existing Executive Drive exit. However, the proposed combined exit is well-spaced from up- and downstream ramps, and this may mitigate the increased volume on this revised ramp. Both frontage road alternatives, while seemingly less disruptive to the freeway than the braided-ramp alternative, will increase volumes at the Foothills Drive ramp-terminal intersection. Traffic operations and queuing should be evaluated to assess queue length on the frontage road and to determine lane configuration needs. 154 Guidelines for Ramp and Interchange Spacing

FIGURECASE STUDY 5 — PROPOSED RAMP SEQUENCE ON I-233 WESTBOUND 5-4 BRAIDED RAMP ALTERNATIVE Airport Foothills Drive Foothills Drive Executive Drive Executive Drive DOUBLE EXIT ALTERNATIVE SINGLE EXIT ALTERNATIVE Airport Airport Executive Drive Executive Drive and Foothills Drive No Lane Balance 2 Lane Changes Required (NO SCALE) N Appendix A 155

Basic Capacity Considerations —All Scenarios None of the scenarios appear to have volumes that are high enough to result in failing operation regardless of ramp spacing. The highest freeway volume on I-233 in all scenarios will be 7,500 vehicles per hour, or 1,875 vehicles per hour per lane (veh/hr/ln) upstream of Green Road. The capacity of a basic freeway segment under ideal conditions is 2,250 to 2,400 veh/hr/ln. I-233 does not have ideal conditions—total ramp density is greater than one ramp per mile, the free-flow speed is unknown, the driver population is less than 1.00, and the heavy vehicles account for 10% of the traffic volume. The basic segment of I-233 upstream of Green Road may be under capacity, although a complete HCM analysis is needed to determine this with certainty due to the non-ideal conditions that exist. The highest volume ramp in any scenario is the frontage road ramp in the latter two alternatives, with 900 vehicles per hour. This value is less than half the capacity of a ramp roadway. Of greater concern operationally than ramp roadways are ramp-freeway junctions. The maximum desirable flow rate entering a merge influence area is 4,600 passenger cars per hour, and the maximum flow rate entering a diverge influence area is 4,400 passenger cars per hour. However, determining the number of vehicles in an influence area (the right two lanes of the freeway and the ramp itself) requires a complete HCM analysis. Summar y The complexity of all three alternatives diminishes the value of applying planning-level operational analysis tools. However, it appears that the braided-ramp alternative will have the most operational impact on the freeway due to the weaving section requiring two lane changes, which is preceded by several other closely spaced ramps. The double-exit-with- frontage-road alternative eliminates the weaving section, and the single-exit - with-frontage-road alternative eliminates the weaving section and another closely spaced ramp combination. STEP 3—Safety: Table 5-1 summarizes the ramp combinations of interest for the three alternatives in this case study. Research conducted to develop thes e Guidelines did not show an increase in crashes associated with a decrease in ramp spacing for the EX-EN ramp combination. Limitations of these findings are identified in Guidelines Section 4.5.4.3. The EX-EX combination was not studied from a safety perspective (in research conducted for the Guidelines ) , but results are expected to be consistent with the EX-EN results (i.e., no relationship between ramp spacing and safety). Basic Capacity Thresholds from the HCM are presented in Table 4-1 of the Guidelines. The ramp-braid concept could possibly be dropped at this point in the evaluation. However, for the purpose of this case study, it will be carried forward. The EX-EN ramp combination spaced at 2,000-2,200 feet as part of the braided ramp alternative is not expected to cause a freeway mainline safety issue. Research used to draw this conclusion is limited. The geometric analysis discussed in Guidelines Section 5.3.1.4 should be a primary factor in the spacing assessment until additional safety information becomes available. 156 Guidelines for Ramp and Interchange Spacing

Quantitative safety conclusions for the EX-EX ramp combinations spaced between 1,400-1,600 ft for the braided ramp alternative and 1,000-1,400 ft for the double exit/frontage road alternative cannot be drawn using existing safety research. Again, freeway mainline safety issues are not expected as a result of the tighter spaced EX-EX combinations if limited findings for the EX-EN can be generalized to the EX-EX. However, future research to explore the EX-EX combination is needed. Guidelines Exhibit 5-5 indicates that the 1,300-1,900 ft EN-EX combination that is part of the braided ramp alterative may result in 8 to 19% more mainline crashes than a basic freeway segment of the same length. This result is found by subtracting the relative crash risk for a long spacing (which approaches -12% for spacing dimensions greater than 3,000 ft) from the relative crash risk for the 1,300-1,900 ft range (7 to -4%). STEP 4—Signing: Signing should be considered at the earliest stages of concept development to assess the types and amount of information that will need to be presented and to consider the advance placement of signs. This is especially true for complex highway and interchange configurations such as those presented here. The Executive Drive/Foothills Drive/airport interchange area is near the Green Road and Sunset Street/Oak Street interchanges. Signing needs for these interchanges should be incorporated into the signing assessment for each of the concept alternatives for the new Foothills Drive ramps. Existing signing on this portion of I-233 westbound is shown in Figure 5-5. All signs are overhead due to the number of lanes on the freeway, and future signs should be overhead as well. The advance guide sign sequence for most exits begins more than one mile before the exit due to the high number of drivers on I-233 who are unfamiliar with the area and the importance of some of the interchanges. It is desirable to maintain this advance signing. At one location along I-233 between Green Road and Executive Drive, three advance guide signs currently exist. This is the maximum number recommended by the MUTCD, and collectively they contain the maximum number of message units recommended by the ITE Handbook. This is with more information than they are able to process. The EX-EX spacing values of 3,800-4,200 feet for the double exit/frontage road alternative and 5,200 feet for the single exit/frontage road alternative are not expected to reduce freeway mainline safety. For EN-EX and EN-EN ramp combinations, mainline safety approached that of a basic freeway segment when ramp spacing values were greater than 3,000 feet. The conclusion is generalized to the EX-EX alternative until additional research is available. Without quantitative safety findings, the geometric analysis (Section 5.3.1.3) and signing considerations (Section 5.3.4) are the primary factors for the EX-EX spacing assessment. computed in Table 5-4. Adding new exit ramps could potentially necessitate a fourth guide sign at this location or other locations that could present drivers Appendix A 157

Table 5-4 Computation of message units at existing sign assembly on I- 233 westbound between Green Road and Executive Drive 4 message units • Exit number • Road Name • Second Road Name • Distance 4 message units • Exit number • Destination • Connecting Route • Distance 3 message units • Exit number • Road Name • Distance 3 sign panels – maximum recommended 11 total message units – maximum recommendation Braided Ramps The braided-ramp scenario adds an exit ramp for Foothills Drive to I-233. The gore will be located near the Executive Drive overpass where a sign for the airport exit and a pull-through sign for I-233 are currently located. The MUTCD discourages placing signs other than an exit direction sign near the gore, so the existing signs should be moved downstream. Ideally, the signs will be located in the following places: • At least 800 ft beyond the Foothills Drive exit direction sign (recommended by the MUTCD); • At least 800 ft beyond the Executive Drive overpass so that the bridge structure does not obscure drivers’ view of the sign (recommended by the MUTCD); • At least a quarter of a mile upstream of the airport exit/pull though sign assembly that is at the gore of the airport exit; and, • At a location that will minimize the attentive demands on the driver. A sign placement concept developed with these considerations in mind is shown in Figure 5-6. Its development is discussed in detail below. Before beginning to place signs for Foothills Drive, existing signs that will be in the vicinity of the exit gore should be moved and modified if necessary. Based upon MUTCD recommendations, the airport exit and pull-though sign assembly can be moved from their current location to a position that is 800 ft or more downstream of the Foothills Drive exit direction sign, but still upstream of the weaving segment between the Executive Drive onramp and the airport exit. To reduce the number of message units in an area with many signs, “west to I-33” can be removed from the I-233 pull-though sign. This information can be conveyed to drivers elsewhere in the corridor. The number of the airport exit will need to be changed from 17 to 17B. The new Foothills Drive exit will be located between exits 17 and 18. Since it will be closer to mile marker 17 than mile marker 18, it will be numbered as exit 17A. The expected difference between freeway mainline crashes for the 1,300 to 1,900 foot EN-EX combination that is part of the braided ramp alternative and a basic freeway segment can be reduced or eliminated if an auxiliary lane is provided. Moving existing signs and eliminating unnecessary information will help to accommodate new signs for the new interchange. Exiting numbers must also be changed to serve the proposed new exit. 158 Guidelines for Ramp and Interchange Spacing

Appendix A 159

Now that existing signs have been moved based upon the location of the new ramp, placement of advance guide signs for Foothills Drive may occur. Three-quarters of a mile upstream of the new Foothills Drive exit gore is an overhead sign assembly with panels for the airport exit, the Executive Drive exit, and the Sunset Street/Oak Street exit. This assembly is the natural location for an advance guide sign for Foothills Drive. It is not too far from the Foothills Drive exit, nor is it in the vicinity of the exit direction sign for Executive Drive. Since the assembly already has three sign panels, the one for the furthest exit (Sunset Street/Oak Street) should be removed when the Foothills Drive advance guide sign is added. An interchange sequence sign is not appropriate here because such a sign would not be able to indicate that the airport exit goes to SR 67 south and, per MUTCD recommendations, could not contain more than three exit names and distances (i.e., Sunset Street/Oak Street could not be included anyway). A 1¼-mile advance guide sign for Foothills Drive can be added to the sign assembly mounted on the Green Road overpass, and a two-mile advance guide sign for Foothills Drive can be added to the sign assembly that is one- half mile upstream of the Green Road exit. These additions would place three sign panels at each location. Under such a circumstance, interchange sequence signs could also be used. However, the MUTCD recommends that interchange sequence signs be used over the entire length of a route in an urban area, and on I-233 they would only be needed in these two locations. All sign placements associated with the braided-ramp alternative are shown in Figure 5-6. The braided-ramp alternative will require signing that begins to exceed the number of message units that drivers are able to comprehend and process. Three sign assemblies in the corridor will display three guide signs each. The Sunset Street/Oak Street interchange will not be signed until one-half mile in advance of the exit, since sign assemblies for several miles upstream cannot accommodate additional sign panels. There may also be challenges locating the exit direction sign for Foothills Drive since it will be near an overpass. Based on the signing alone, the braided-ramp alternative is not recommended. Double Exit with Frontage Road This alternative has signing similar to the braided-ramp alternative. No sign layout is provided for this alternative because of its similarity to the braided- ramp alternative shown in Figure 5-6. The same number of exit ramps will exist at approximately the same locations as under the braided-ramp scenario. Many of the same issues that exist with the braided-ramp alternative, such as a high number of message units, also exist with this alternative. However, from a signing perspective, it is superior to the braided-ramp alternative in two ways: Advance guide signs for Foothills Drive will need to be placed upstream of the exit. Existing sign assemblies will be the logical locations to add these signs. The location of additional advance guide signs can be determined in the same manner as the ¾-mile sign. It does not appear that the braided-ramp alternative can be adequately signed. The configuration requires signing that exceeds the number of message units that drivers are able to comprehend and process. There may also be challenges locating the exit direction sign for Foothills Drive since it will be near an overpass. Based on the signing challenges, the issues with lane balance and weaving on the mainline, and the previously identified operational issues, this alternative is a strong candidate for elimination from further consideration. Appendix A 161

162 Guidelines for Ramp and Interchange Spacing

• Without the Executive Drive onramp, there is more flexibility in locating the Foothills Drive exit ramp. This flexibility can be used to minimize the impact of placing the sign for the exit so near the overpass. • Without the Executive Drive onramp, there will be no auxiliary lane immediately prior to the airport exit. There will not be a change in lane configuration for the airport exit on the two signs prior to it. Despite the positive signing qualities of this configuration compared to the braided-ramp alternative, the double-exit-with-frontage-road alternative is not recommended because it would require signing that exceeds the number of message units drivers can be expected to comprehend. Single Exit with Frontage Road The single-exit-with-frontage-road alternative has one less exit than the other two alternatives and will be considerably easier to sign as a result. There will be no exit gore near the Executive Drive overpass and, thus, no signing issues relates to this. Signs for the airport exit downstream of Executive Drive do not need to be modified or relocated. The Sunset Street/Oak Street exit, while not signed the typical one or two miles prior to the exit, can be signed 2 ¾ miles prior to the exit. A sign layout for this alternative is shown in Figure 5-7. In this alternative there is only one location that requires three sign panels. At this location, which is between Green Road and Executive Drive, each sign will have four message units: an exit number, two street names, and the distance to the exit. To reduce the number of message units, the “To SR 67 South” text on the sign for the airport exit could be eliminated. Other signs for the airport indicate that SR 67 can be accessed from the airport exit. Signing at this assembly will essentially be at but not exceed the limit of what drivers are able to process from a message-unit perspective. Other Considerations The proposed interchange will be on the Interstate Highway System, and therefore, changes must be approved by FHWA. Discussions between the state transportation agency and FHWA are underway, and FHWA has indicated it will approve the proposed interchange if a traffic study demonstrates that it meets the requirements of the agency’s access review policies. The single exit with frontage road alternative is superior to the other two from a signing perspective. 164 Guidelines for Ramp and Interchange Spacing

Appendix A 165

FINDINGS Adding an exit and entrance ramp to serve Foothills Drive from I-233 westbound will be challenging due to ramp spacing issues. The use of braided ramps between Executive Drive and Foothills Drive is not recommended. It will keep the existing weaving section prior to the airport diverge (which violates the principles of lane balance and requires two lane changes for some movements) intact, and sufficiently signing all exits without overloading drivers with too many message units will not be possible. The double-exit- with-frontage-road alternative eliminates the weaving section prior to the airport diverge, but still cannot be adequately signed. The single-exit-with- frontage-road alternative is preferred from a spacing perspective, as it does not have any of the issues noted above. Ramp spacing dimensions are well above the AASHTO policy’s recommended minimums, major operational impacts are not anticipated, and signing needs can be satisfied. Significant freeway mainline safety impacts of a new Foothills Drive interchange are unlikely. Research used to draw safety conclusions related to some of the tightly spaced ramp combinations is limited. The braided-ramp alternative is a candidate for early screening and elimination from further study for the reasons noted above. The double-exit- with-frontage-road alternative is as well, unless it has significant advantages over the single-exit-with-frontage-road alternative for aspects of the project not related to ramp spacing, such as arterial operations, cost, environmental studied in greater detail. constrains, etc. The single-exit-with-frontage-road alternative should be Appendix A 167