Guidelines for Ramp and Interchange Spacing (2011) / Chapter Skim
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92 Appendix A Scenario Based Case Studies
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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.
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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.
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FIGURE 56 32 248 < 2 2 > At-grade intersection to be converted to a diamond interchange N CASE STUDY 1 SITE MAP 1-1
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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.
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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)
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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.
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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.
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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.
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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.
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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.
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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
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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
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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)
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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.
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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.
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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.
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• 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.
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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.
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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.
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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.
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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.
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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 )
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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.
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the time the back of queue is reached. The FFS of the freeway can be used for a conservative design.
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freeway's basic lanes)
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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.
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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.
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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
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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
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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)
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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.
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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.
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Appendix A 127
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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.
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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)
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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
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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)
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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.
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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
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Source: Digital Globe 134 Guidelines for Ramp and Interchange Spacing
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Based on the ramp dimensions noted previously (1,300-1,500 ft and 1,4001,800 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.
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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)
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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.
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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)
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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.
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Ramp spacing values currently are 2,200 (westbound) and 2,800 ft (eastbound)
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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)
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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.
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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.
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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
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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.
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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.
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CASE STUDY 5 -- PROPOSED RAMP SPACINGS AND TRAFFIC VOLUMES (BRAIDED RAMPS ALTERNATIVE) FIGURE 5-3A 7,300600 Notes: 1.
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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.
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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.
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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.
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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.
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between Executive Drive and the new frontage road. The frontage road would then connect to Foothills Drive.
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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)
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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.
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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
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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)
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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.
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Table 5-4 Computation of message units at existing sign assembly on I233 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.
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Appendix A 159
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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.
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162 Guidelines for Ramp and Interchange Spacing
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• 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.
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Appendix A 165
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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.
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