Access Management in the Vicinity of Interchanges, Volume 2: Research Overview (2022)

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101   Conclusions This final report for NCHRP Project 07-23 includes a review of the published literature and current state agency practices regarding the spacing of access points on the crossroad in the vicinity of interchanges (see Chapter 2). The research that focuses on this topic is quite limited, and much of it was previously captured in a synthesis study (6). In addition, this report documents the research team’s summary of identified gaps in research. The project team identified these gaps using a variety of resources, including the published literature, a review of current state practices, and feedback from access management pro- fessionals. Furthermore, NCHRP Synthesis 332 included additional recommendations about research needs focused on better understanding of access management on crossroads near interchanges (6). Based on the identified gaps and panel approval of the work plan, the project team acquired extensive field data (as summarized in Chapter 3). This data collection effort included two different time periods. The originally funded research included data for 16 interchange locations. As the original research neared completion, the panel requested supplemental findings that ultimately enabled the team to acquire data at 10 additional signalized diamond interchange locations. The addition of these diamond interchange locations, coupled with micro-simulation to field validate the collected data, enabled the research team to expand the signalized diamond statistical analysis to provide more robust, reliable relationships than were previously feasible. The conclusions noted in this chapter, therefore, address the following items: • Overview of operational performance findings, • Overview of safety performance findings, • Sample guideline language, and • Recommendations for future research. Operational Performance Findings Based on a combination of detailed field operational observations and micro-simulation efforts, the research team explored the role that the various key access management features had on the corridor speed at locations near interchanges. Because speed can be expected to differ for vehicles driving away from the interchange when compared to vehicles approaching the intersection, the research team developed statistical models to assess the average corridor operating speed as well as the speed departing and approaching the terminal intersections at the individual interchanges. Highlights of this analysis are summarized as follows (refer to Table 54 for the specific numeric relationships). • Signalized diamond interchanges without channelized right turns: – As volume increases, the average operating speed decreases. – As the distance to the nearest signalized intersection increases, the average speed increases. C H A P T E R 6

102 Access Management in the Vicinity of Interchanges – An increase in posted speed limit has a positive relationship to the average speed for both directions of travel and the direction of travel approaching the interchange. The posted speed limit does not strongly influence the average speed for the direction of travel depart- ing the interchange. – The presence of a raised median is positively linked to an increase in speed for vehicles traveling toward the interchange. In contrast, the presence of a two-way left-turn lane can be expected to result in a reduction in average travel speed on this approach. – An increase in the number of access points is associated with a reduction in the average speed for both directions of travel as well as for the average speed as vehicles approach the interchange. The distance to the USDW is the access point that influences the average speed away from the interchange. • Signalized diamond interchanges with channelized right turns: – Average speed for both directions, average speed for the direction departing the inter- change, and average speed for the direction approaching the interchange are all influenced by an increase in volume (equated to a reduction in speed) and an increase in the distance to the downstream signal (increasing the average speed for both directions and the average speed toward the interchange). The average speed away from the interchange has a nega- tive relationship. – An increase in the access density will result in a reduction in the average speed for the lanes with vehicles traveling away from the interchange as well as for the average speed for both directions of travel. The access density does not have a significant influence on the average speed toward the interchange. • STOP-controlled diamond interchanges: – For all three average speed metrics, the average speed decreases with an increase in volume and access density. – The average speed increases with an increase in the distance to the nearest DSDW. • Partial cloverleaf interchanges (PARCLOs): – The type of PARCLO appears to have a direct influence on speed estimation. The average speed for all three metrics will decrease as the volume increases. – Other influential factors for a PARCLO include median options (for average speed in both directions and average speed toward the interchange), distance to a signalized intersection (for average speed in both directions and average speed away from the interchange), and distance to a downstream driveway (average speed away from the interchange and average speed toward the interchange). • Single-Point Urban Interchanges (SPUIs): – For all three average speed metrics, the average speed decreases as the volume increases, while the speed can be expected to increase as the distance to the DSDW increases. – An increase in the distance to the nearest signalized intersection equates to an increase in the average speed for both directions of travel as well as for the average speed toward the interchange. Note that this distance is negatively associated with the average speed for vehicles traveling away from the interchange. – As access density increases for the average speed in both directions and average speed away from the interchange, the speed values decrease. Access density, however, was not significant for average speed toward the interchange. – As observed for the PARCLO configuration, the SPUI is also characterized by the presence of a median treatment; however, the resulting values are counterintuitive. This observation may be due to the limited number of study sites and merits future evaluation. Safety Performance Findings Due to the relatively small sample of study sites, the development of empirical safety pre- dictive models is not feasible. To overcome this limitation, team members reviewed crash data,

Conclusions 103   collision trends, and common conflicts between users of the facility. The following observations resulted from this evaluation: • The most commonly observed crash types were rear-end and angle crashes. These are commonly observed at intersection locations; therefore, these crash types can be expected for this study. • The number and severity of crashes ranged widely, but most crashes were possible injury or property damage only. • Several locations appeared to be subjected to pedestrian and bicycle crashes. These crashes frequently occurred at driveways located between the terminal intersection and the next signalized intersection away from the interchange. • The placement of the first access point near the terminal intersection generally has a larger influence on corridor safety. For several facilities, this access point placement also strongly influenced operational performance (see previous discussion). • Queues that form upstream of a signalized terminal intersection can be expected to provide a safer solution if a raised median is located adjacent to the queued region. This solution will limit erratic lane changes by motorists exiting driveways on the opposite side of the road. • When feasible, drivers turning right from the terminal intersection, who then intend to turn left at the next intersection, can be observed not to turn into the closest lane. This type of maneuver can be limited using intersection traffic control treatments as well as physical roadway channelization. Sample Guideline Language The research team evaluated potential language updates or changes for the AASHTO Green Book, the TRB AMM, and the AASHTO HSM. Due to the limited empirical safety analysis, team members were not able to develop substantive language for the HSM. Suggested language for the other two national documents is summarized in this section. Suggested Language for the AASHTO Green Book Green Book, page 2-44, 1st bullet: • Existing wording: Direct property access should be denied or limited along higher-class roadways whenever reasonable access can be provided to a lower-class roadway. • Suggested expansion: Direct property access should be denied or limited along higher-class roadways whenever reasonable access can be provided to a lower-class roadway. At urban arterial locations in the vicinity of interchanges, the first driveway adjacent to an interchange terminal intersection should exceed corner clearance criteria wherever possible. Driveways located within less than 1,000 ft of the intersection should be complemented with a raised median that will further restrict driveway access from opposing traffic. Green Book, page 10-8, 6th paragraph: • Existing wording: In urban areas, high turning volumes and close spacing between adjacent ramp terminals and access connections may result in congestion on the crossroad that affects traffic on the ramp and may spill back onto the freeway. • Suggested modification: In urban areas, high turning volumes and close spacing between adjacent ramp terminals and access connections may result in congestion on the crossroad. The average speed and associated congestion can be managed by limiting the number of access points, extending the distance to the nearest signalized intersection, lengthening the distance to the closest upstream and downstream driveways, and channelizing traffic through

104 Access Management in the Vicinity of Interchanges the use of a raised median or driveway directional turnout. The implementation of these effective access management strategies will improve traffic operations on the arterial cross- road and the interchange off-ramps. Suggested Language for the TRB AMM AMM, page 433 bullet list: • Existing wording: There are several critical locations downstream of an off-ramp. These locations include: – The first driveway after an interchange off-ramp terminal, – The first unsignalized crossroad intersection after an interchange off-ramp terminal, – The first median opening after an interchange off-ramp terminal, and – The first signalized crossroad intersection after an interchange off-ramp terminal or the closest access drive. • Suggested expansion: Retain the text shown above and at the end include the following: These four critical locations are not independent and should be considered collectively. For example, as the distance to the first driveway increases, the distance to the first unsignalized crossroad should not be reduced. Of particular importance is the placement of the raised median near the ramp terminal. The median should restrict access so that vehicles do not turn right and then traverse lanes to immediately turn left into a driveway or unsignalized intersection. The median placement at this location will channelize traffic and restrict vehicle maneuvers that create operational and safety challenges. Future Research Recommendations As a result of this research effort, several items could benefit from additional research. The type of median and its influence on speed, for example, appear to vary for target speed. An expanded study of PARCLOs and SPUIs could help to determine why this influence appears to vary by interchange type. Because there are characteristics that are more descriptive for a PARCLO than for other interchange forms, the team recommends additional in-depth research related to PARCLOs, their orientation, and safety and operational components. Final Comments This report concludes with a list of references followed by appendices that include a comprehensive summary of the key data for each of the study sites. In addition, the project deliverables include NCHRP Research Report 977: Access Management in the Vicinity of Inter- changes, Volume 1: Practitioner’s Guide, a companion spreadsheet tool, and training modules. NCHRP Research Report 977, Volume 1 was developed as a concisely worded resource that can be used by practitioners who are tasked with developing an access management plan for arterials near interchanges. A series of case studies, designed to help provide some contextual structure for applying the findings of this research, are included in the Guide. In addition, the case studies have been developed using self-calculating spreadsheet tools that can be used for evaluating candidate corridor scenarios. Finally, the training modules step transportation decision-makers through the process of applying effective access management strategies for locations near interchanges.