How to Measure and Communicate the Value of Access Management (2023)

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101   A P P E N D I X B Methods to Measure the Operational Value of Unsignalized Access Spacing Access points introduce conflicts and friction into the traffic stream. As stated in A Policy on Geometric Design of Highways and Streets (AASHTO, 2018), “Driveway terminals are, in effect, low-volume intersections; thus, their design and location merit special consideration.…” As a result, spacing standards for unsignalized access points are needed for managing access to protect the operations and safety of a roadway. Methods to measure the value of unsignalized spacing for mobility are provided in the following sources: • Highway Capacity Manual, 6th ed. (HCM6, 2016), Chapter 18 (Urban Street Segments), Section 3 (Motorized Vehicle Methodology): Estimates the increase in delay and the reduction in roadway free-flow speed due to increased access point density. • Access Management Manual (Williams et al., 2014): Estimates the impact on blockage of a driveway due to it being located too close to a signalized intersection. • NCHRP Report 420: Impacts of Access Management Techniques (Gluck et al., 1999): Estimates the mobility impacts of unsignalized access spacing on through traffic in the curb lane. • HCM6 (2016), Chapter 18 (Urban Street Segments), Section 5 (Bicycle Methodology): Estimates the reduction in delay due to increasing unsignalized access spacing and decreasing access density. Delay Due to Access Density The reduction in roadway free-flow speed due to increased access point density may be estimated using Table A2 from NCHRP Research Report 900 (Butorac et al., 2018a), which was adapted from Exhibit 18- 11 of HCM6 (2016). The speed reduction (in mph) associated with the increased access points equals −0.078 Da/Nth, where Da is the number of access points per mile (considering both sides of the roadway, but only those accessible to or from the direction of travel) and Nth is the number of through lanes in the direction of travel. The magnitude of the corresponding change in average running speed will be slightly lower, as discussed in the appendix of NCHRP Research Report 900 (Butorac et al., 2018a). Exhibit 18-13 from HCM6 may be used to estimate the average through-vehicle delay in terms of seconds per vehicle per full, unsignalized access point. Delay values in Exhibit 18-13 assume 10% of the traffic on the street turns right at the access point and 10% turns left (HCM6, 2016). The delay values need to be adjusted proportionately for other turning percentages. In addition, the delay values need to be reduced by 50% if one turning movement is provided with an appropriately dimensioned turn lane or the turning movement does not exist. There is no delay if both turning movements are provided with turn lanes (or if one movement has a turn lane and the other movement does not exist) (Butorac et al., 2018a).

102 How to Measure and Communicate the Value of Access Management Source: Exhibit 18-13, HCM6, 2016. Procedures that may be used to quantify the following mobility impacts related to unsignalized access spacing include (Gluck et al., 1999): • Vehicles “impacted” by a single driveway and by multiple driveways. • Influence area lengths, including spillback implications across upstream driveways. The percentage of through vehicles in the right (curb) lane that would be affected at a single driveway may be estimated. The percentage of through vehicles affected increases as right-turn volumes increase, as shown in Table 11. The percentage of right-lane through vehicles that would be affected at least once per ¼mile is shown for different access spacings in Table 12 (Gluck et al., 1999). Table 11. Percentage of through vehicles affected at one driveway as right-turn volume increases. Source: Table 8, Gluck et al., 1999. Table 12. Percentage of right-lane through vehicles affected at least once per ¼ mile. Source: Table 9, Gluck et al., 1999. The influence distances were calculated by adding driver perception-reaction distances and car lengths to the effect lengths. Using this information, the percentages of right-lane through vehicles that would be Table 10. Delay due to turning vehicles.

Methods to Measure the Operational Value of Unsignalized Access Spacing 103   influenced to or beyond an upstream driveway in a ¼-mile section may be estimated for various right-turn volumes, driveway spacings, and posted speeds. As an example, the likely percentages of affected vehicles that would extend to or beyond at least one driveway (upstream) per ¼mile (i.e., “spillback”) for a 45-mph speed are shown in Table 13. This information may be used to identify the cumulative effect of decisions concerning driveway locations and unsignalized access spacing (Gluck et al., 1999). Table 13. Percentage of affected vehicles. Source: Table 10, Gluck et al., 1999. Bicycle Mobility Impacts of Access Spacing The effect of unsignalized access spacing on bicycle level of service (LOS) may be estimated using Equations 18-46 and 18-47 from HCM6 (2016). With this methodology, there is no effect when the access density (total of both sides) is 20 access points per mile or less. Decreasing the access point density by 10 points per mile (e.g., from 30 to 20 points per mile) improves bicycle LOS by 0.14 point while decreasing the access point density by 20 points per mile improves bicycle LOS by 0.28 point. These results assume that heavy vehicles make up 5% of the traffic volume and that roadway links and signalized intersections are weighted the same when calculating overall bicycle LOS (Butorac et al., 2018a).

Abbreviations and acronyms used without denitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FAST Fixing America’s Surface Transportation Act (2015) FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration GHSA Governors Highway Safety Association HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S. DOT United States Department of Transportation

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