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30 Description A desired uniform traffic signal spacing is defined and implemented over time. Techniques in this group influence the spacing of traffic signals along a roadway by managing where a new traffic signal may be installed. Long, uniform traffic signal spacing facilitates the ability to provide two-way traffic progression under a variety of traffic conditions (1, 2). Minimum progression bandwidths are defined as part of the criteria (e.g., to be considered when deviations to the desired spacing are being evaluated). Tables 17 through 21 follow. Quantitative Analysis Methods Motor Vehicle Operations Chapter 16 in the HCM6 (4) can be used to estimate the travel time impacts of traffic signal spacing and location on motor vehicle travel times. NCHRP Report 420 (3) provides estimates of the percent increase in travel times when traffic signal spacing is greater than two signals per mile (see Table 20). Motor Vehicle Safety NCHRP Report 420 reported average crash rates (crashes per million vehicle miles) by different ranges of traffic signal densities (3) (see Table 21). C H A P T E R 6 Traffic Signal Spacing Source: Photograph provided by the authors.
Traffic Signal Spacing 31 Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Lengthen traffic signal spacing. â â â â â â â â Â Â Â Â Â Â Â Â Â Locate new driveway opposite existing signalized driveway. â Â Â Â Â â Â Â Â Â Â Â Â Locate new high-volume driveways where signal spacing criteria can be met. â â â Â Â â â Â Â Â Â Â Â Â Design driveways and medians such that signals only affect one side of arterial at a time. â â Â â â â â Â Â Â Â Â Â Â Â Â Â Â Table 17. Multimodal operations and safety performance summary. Mode Operations Safety Motor vehicle travel speeds are higher by 2 to 3 mph with each one-signal-per-mile reduction in the signal density when signals are closely or irregularly spaced (1, 3). The crash rate of the motorized vehicle decreases (1, 3). Small negative effect on pedestrian LOS due to increased motor vehicle speeds (4, 5). When midblock crossings are illegal or experience high delays, pedestrian LOS decreases due to the longer detour required to walk to the nearest signalized intersection (4, 5). Increases in vehicle speeds may negatively affect pedestrian safety (6, 7). Bicycle LOS worsens due to increased motor vehicle speeds, which negatively affect bicycle LOS (4, 5). Average bicycle travel speeds increase with fewer signals per mile (4). Increases in vehicle speeds may negatively affect bicycle safety (6, 7). Similar to motor vehicles, but with greater benefit for buses, as they accelerate more slowly to their running speed after stopping or slowing (8). Schedule reliability improves (9). Increases the need for bus stops between signalized intersections to minimize passenger walking distances; these stops may be difficult for passengers to access by crossing the street (10). No documented effect beyond that generally observed for motor vehicle traffic and pedestrians. Similar to motor vehicles, but with greater benefit for trucks, as they accelerate more slowly to their running speed after stopping or slowing (11). Improved truck LOS due to improved speeds (12). No documented effect beyond that generally observed for motor vehicle traffic. Table 18. General trends associated with longer traffic signal spacing.
32 Guide for the Analysis of Multimodal Corridor Access Management Signals per Mile Crash Rate (crashes per million vehicle miles) ⤠2 3.5 2.01â4 6.9 4.01â6 7.5 > 6 9.1 Table 21. Average crash rates by different ranges of traffic signal densities. Mode Operations Safety Reduces traffic signal delay and number of stops, thus improving overall travel time (4). Improves fuel economy and air quality (6). Helps create gaps in traffic that can be used by turning vehicles (13). The motorized vehicle crash rate decreases (6). Helps create gaps in traffic that can be used by crossing pedestrians. Helps create gaps in traffic that can be used by crossing pedestrians. No documented effect. No documented effect. When bus stops are located on the far side of signalized intersections, buses can take advantage of motor vehicle progression provided along a corridor, resulting in reduced and less variable travel times (9). No documented effect beyond that generally observed for motor vehicle traffic. Similar to motor vehicles, but with greater benefit for trucks, as they accelerate more slowly to their running speed after stopping or slowing (11). No documented effect beyond that generally observed for motor vehicle traffic. Table 19. General trends associated with providing traffic progression. Signals per Mile Increase in Travel Times Relative to Two Signals Per Mile (%) 3 9 4 16 5 23 6 29 7 34 8 39 Table 20. Percent increase in travel times.
Traffic Signal Spacing 33 Data from southeast Michigan indicate that providing signal progression reduces the number of collisions in a corridor by 10% to 20% (6). Pedestrian Operations Equations 18-32 and 18-35 in the HCM6 (4) can determine the effect of motorized vehicle speeds on pedestrian link LOS. An increase in average traffic speed of 2 mph worsens the pedes- trian link LOS score by 0.03 points (at 20 mph) to 0.08 points (at 50 mph), with 0.75 points representing the range covered by one LOS letter. Increasing the traffic signal spacing may reduce pedestrian LOS in situations when no legal pedestrian crossings are available between signals or when the pedestrian delay experienced waiting for a safe gap to cross the street is greater than the time required to detour to the nearest signalized crossing (8). The impact on the pedestrian LOS score depends on the change in delay for pedestrians and the quality of the pedestrian environment along the roadway (âlinkâ) and at signalized intersections. Greater delays and poorer link and intersection pedestrian environments result in greater reductions in the pedestrian LOS score. Chapter 18 in the HCM6 describes the methodology. Bicycle Operations Equations 18-41 and 18-44 in the HCM6 (4) can determine the effect of motorized vehicle speeds on bicycle LOS. An increase in average traffic speed of 2 mph worsens the bicycle LOS score by 0.57 points (at 21 mph or less), 0.17 points (at 25 mph), 0.05 points (at 40 mph), and 0.03 points (at 50 mph), with 0.75 points representing the range covered by one LOS letter. The bicycle methodology in Chapter 18 in the HCM6 can be used to estimate the impact of traffic signal delays on overall bicycle travel times along a roadway (4). Bicycle Safety Carter et al. (14) developed a model to predict a bicycle intersection safety index. The index value for through bicycle movements worsens by 0.428 rating points if a traffic signal were installed and no bicycle lane was present. The index value for left-turning movements worsens by 0.485 ratings points if a traffic signal were installed. See the appendix for more details about this model. Bus Operations Chapter 18 in the HCM6 (4) can be used to estimate the change in average bus speeds resulting from changes in signal spacing and intersection delay. This information in turn can estimate the change in bus LOS for the segment. At 30-minute bus headways and with seated loads and reliable service, a 1-mph increase in average bus speed produces a 0.08â0.12 improvement in the bus LOS score, with 0.75 points representing the range covered by one LOS letter (4, 10). Truck Operations Section P in Exhibit 3 of NCHRP Report 825 (15), derived from NCFRP Report 31 (12), can be used to estimate the effect of improved truck free-flow and travel speeds on overall truck LOS. On a level street with a 35-mph free-flow speed, increasing average truck speeds from 25 to 26 mph results in a 1.3 percentage point increase in the truck LOS index, while increas- ing average truck speeds from 17.5 to 18.5 mph results in a 7.0 percentage point increase, with 10 percentage points representing the range covered by one LOS letter.
34 Guide for the Analysis of Multimodal Corridor Access Management Additional Information ⢠Chapter 12 in this guide. ⢠Access Management Manual, Second ed.: Sections 15.2 and 20.2.1. ⢠Access Management Application Guidelines: Chapter 13, Signalized Access Spacing. ⢠NCHRP Report 420: Chapter 3, Traffic Signal Spacing. References 1. Williams, K. M., V. G. Stover, K. K. Dixon, and P. Demosthenes. Access Management Manual, Second ed. Transportation Research Board of the National Academies, Washington, D.C., 2014. 2. Stover, V. G., and F. J. Koepke, Transportation and Land Development, 2nd ed., Institute of Transportation Engineers, Washington, D.C., 2002. 3. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, Washington, D.C., 1999. 4. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 5. Dowling, R., D. Reinke, A. Flannery, P. Ryus, M. Vandehey, T. Petritsch, B. Landis, N. Rouphail, and J. Bonneson. NCHRP Report 616: Multimodal Level of Service Analysis for Urban Streets. Transportation Research Board of the National Academies, Washington, D.C., 2008. 6. Chandler, B. E., M. C. Myers, J. E. Atkinson, T. E. Bryer, R. Retting, J. Smithline, J. Trim, P. Wojtkiewicz, G. B. Thomas, S. P. Venglar, S. Sunkari, B. J. Malone, and P. Izadpanah. Signalized Intersections Informational Guide, 2nd ed. Publication No. FHWA-SA-13-027. Federal Highway Administration, Washington, D.C., July 2013. 7. National Association of City Transportation Officials. Urban Street Design Guide. National Association of City Transportation Officials, Washington, D.C., Oct. 2013. 8. Hemily, B., and R. D. King. TCRP Synthesis 75: Uses of Higher Capacity Buses in Transit Service. Transporta- tion Research Board of the National Academies, Washington, D.C., 2008. 9. Ryus, P., K. Laustsen, K. Blume, S. Beaird, and S. Langdon. TCRP Report 183: A Guidebook on Transit-Supportive Roadway Strategies. Transportation Research Board, Washington, D.C., 2016. 10. Kittelson & Associates, Inc.; Parsons Brinckerhoff; KFH Group, Inc.; Texas A&M Transportation Institute; and Arup. TCRP Report 165: Transit Capacity and Quality of Service Manual, 3rd ed. Transportation Research Board of the National Academies, Washington, D.C., 2013. 11. Harwood, D. W., D. J. Torbic, K. R. Richard, W. D. Glauz, and L. Elefteriadou. NCHRP Report 505: Review of Truck Characteristics as Factors in Roadway Design. Transportation Research Board of the National Academies, Washington, D.C., 2003. 12. Dowling, R., G. List, B. Yang, E. Witzke, and A. Flannery. NCFRP Report 31: Incorporating Truck Analysis into the Highway Capacity Manual. Transportation Research Board of the National Academies, Washington, D.C., 2014. 13. Antonucci, N. D., K. K. Hardy, K. L. Slack, R. Pfefer, and T. R. Neuman. NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 12: A Guide for Reducing Collisions at Signalized Intersections. Transportation Research Board of the National Academies, Washington, D.C., 2004. 14. Carter, D. L., W. W. Hunter, C. V. Zegeer, J. R. Stewart, and H. F. Huang. Pedestrian and Bicyclist Intersection Safety Indices: Final Report. Report FHWA-HRT-06-125. Federal Highway Administration, Washington, D.C., Nov. 2006. 15. Dowling, R., P. Ryus, B. Schroeder, M. Kyte, T. Creasey, N. Rouphail, A. Hajbabaie, and D. Rhoades. NCHRP Report 825: Planning and Preliminary Engineering Applications Guide to the Highway Capacity Manual. Transportation Research Board, Washington, D.C., 2016.
