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35 Description This group includes a range of techniques to reduce the number of access points along a road- way, increase the spacing between unsignalized access points, or both. The minimum distances along a roadway between two successive unsignalized connections and between roadway inter- sections and the nearest access point (i.e., corner clearance) are established and implemented over time. The relative locations of access points on opposite sides of the roadway can also be established. Tables 22 through 26 follow. Quantitative Analysis Methods Motor Vehicle Operations Equations 18-3 and Exhibit 18-11 in the HCM6 give the reduction in roadway free-flow speed due to access point density (2). This reduction (in mph) equals â0.078 Da/Nth, where Da is the number of access points per mile (considering both sides of the roadway, but only those acces- sible to or from the direction of travel) and Nth is the number of through lanes in the direction of travel. The resulting increase in average travel speed will be slightly lower, as discussed in the appendix. Exhibit 18-13 in the HCM6 gives average through vehicle delay in terms of seconds per vehicle per full, unsignalized access point (2) (see Table 25). C H A P T E R 7 Number and Spacing of Unsignalized Access Points Source: Photograph provided by the authors.
36 Guide for the Analysis of Multimodal Corridor Access Management Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Increase the spacing between adjacent access points. â Â Â Â Â â â â â Â Increase corner clearance. â Â Â â Â â Â Â Â Â Â Consolidate driveways. â â â â â â â â Â Â Â Â Â Â Â Coordinate driveways on opposite sides of roadway. â Â Â Â Â â Â Â Â Â Â Provide connections between adjacent properties. â â â Â Â â â â Â Â Â Â Â Require access on collector (if available) in lieu of arterial. â â â â â â â â Â Â Â Â Â Â Relocate or reorient access. â Â Â â Â â Â Â â Â Â Â Â Â Â Â Â Â Â Â Â Â Â ÂÂ Â Â Â Â Â Â Â Note: â = unchanged performance. Table 22. Multimodal operations and safety performance summary. Mode Operations Safety Motor vehicle free flow and travel speeds increase by up to a few miles per hour. Speed increases are greater when the number of through lanes is less, when through traffic volumes are higher, and when turning traffic volumes are higher (1, 2). Longer distances between access points provide space to provide turn lanes (3, 4). The motorized vehicle crash rate decreases (5, 6). The number of vehicleâvehicle conflict points decreases. Small negative effect on pedestrian LOS due to increased motor vehicle speeds (2, 10). The number of vehicleâpedestrian conflict points decreases (4). Increases in vehicle speeds may negatively affect pedestrian safety (8, 9). Bicycle LOS improves due to the reduction in the number of access points. This improvement is partially (or wholly, at traffic speeds less than 25 to 30 mph) offset by the increase in motor vehicle speeds, which negatively affects bicycle LOS (2, 7). The number of potential vehicleâbicycle conflict points decreases (4). Increases in vehicle speeds may negatively affect bicycle safety (8, 9). Similar to motor vehicles but with greater benefit for buses, as they accelerate more slowly to their running speed after stopping or slowing (10). Greater flexibility for selecting bus stop locations (4). 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). Improved truck LOS due to improved speeds (12). No documented effect beyond that generally observed for motor vehicle traffic. Table 23. General trends associated with reducing the number of unsignalized access points.
Number and Spacing of Unsignalized Access Points 37 Mode Operations Safety Longer corner clearances reduce the chance of an access being blocked due to downstream intersection queues (4). Longer distances between access points provide sufficient distance for motorists exiting an access to maneuver to make a turn at a downstream intersection or crossover (13). Inter-parcel connections reduce driving time between adjacent sites and potentially reduce the need to drive between sites. Longer access spacing minimizes the number of locations motorists must monitor at a given time (4) and avoids multiple access connections to a single right-turn lane that make it unclear which access a vehicle will turn into (3). On undivided roadways or roadways with a two- way left-turn lane, coordinating access points on opposite sides of the roadway avoids left-turn conflicts between vehicles traveling in opposite directions and avoids jog maneuvers when crossing the roadway (3). As many as half of the crashes within the functional area of an intersection may be driveway-related (5, 6); these can be reduced or eliminated by using corner clearance standards. Providing inter-parcel pedestrian connections reduces travel time between adjacent sites and can reduce traffic on the arterial. With longer access spacing, motorists have fewer distractions and can focus on activity occurring at a given access, including pedestrians crossing or approaching the access. Inter-parcel pedestrian connections can reduce the number of required driveway and aisle crossings while traveling between adjacent sites (4). Providing inter-parcel bicycle connections reduces travel time between adjacent sites and can reduce traffic on the arterial. With longer access spacing, motorists have fewer distractions and can focus on activity occurring at a given access, including bicyclists crossing or approaching the access. Inter-parcel bicycle connections allow travel on lower- volume roadways while traveling between adjacent sites (4). Corner clearance standards can be developed to incorporate sufficient space for a bus stop at intersections, providing convenient access for bus passengers. Access spacing standards can ensure that sufficient space is provided between driveways to develop bus stops that do not block sight distance from driveways when the bus stop is occupied (4). Similar to motor vehicles. Similar to motor vehicles. Table 24. General trends associated with managing access spacing and location. Midsegment Volume (vehicles per hour per lane) Through Vehicle Delay (seconds per vehicles per access point) by Number of Through Lanes 1 Lane 2 Lanes 3 Lanes 200 0.04 0.04 0.05 300 0.08 0.08 0.09 400 0.12 0.15 0.15 500 0.18 0.25 0.15 600 0.27 0.41 0.15 700 0.39 0.72 0.15 Table 25. Average through vehicle delay in terms of seconds per vehicle per full, unsignalized access point.
38 Guide for the Analysis of Multimodal Corridor Access Management Delay values in Table 25 assume 10% of the traffic on the street turns right at the access point and 10% turns left. Adjust the delay values proportionately for other turning percent- ages. Reduce the delay values by 50% if one turning movement is provided with an appro- priately 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). See Chapters 9 and 10 for additional information specific to turn lanes. The Access Management Manual, Second ed. identified the percentage of cycles during which a driveway in proximity to a signalized intersection will be blocked (3, 14) (see Table 26). Motor Vehicle Safety Drawing from a review of a number of crash studies, NCHRP Report 420 (6) indicated that each additional access point per mile increased a roadwayâs crash rate by 4%, relative to the crash rate experienced at 10 access points per mile (total of both sides). Thus, a road with 60 access points per mile would be expected to have 200% more (i.e., three times as many) crashes as one with 10 access points per mile. The HSM provides the following crash modification factors related to urban and suburban arterials in Table 13-58 (5): â¢ Reducing driveways from 48 to 26â48 per mile: 0.71 â¢ Reducing driveways from 26â48 to 10â24 per mile: 0.69 â¢ Reducing driveways from 10â24 to less than 10 per mile: 0.75 A recent study in South Carolina (15) found that increasing the spacing between driveways decreases the crash rate at driveways. Pedestrian Operations Equations 18-32 and 18-35 in the HCM6 (2) 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. Unsignalized access spacing was not found to be a significant predictor of ratings of pedestrian LOS (7). Flow in Lane Adjacent to Driveway (vehicles per hour) Duration of Red Phase (seconds) Percentage of Cycles, by Corner Clearance 25 feet 50 feet 75 feet 100 feet 125 feet 200 15 20 5 1 na na 25 40 16 5 na na 35 58 31 13 5 2 45 71 46 24 11 4 400 15 50 23 9 3 1 25 77 53 30 15 6 35 90 75 55 35 20 45 96 88 74 56 38 Note: Assumes that the average vehicle length, including the space between stopped vehicles, is 25 feet; na = not applicable. Source: Access Management Manual, Second ed., Exhibit 15-39 (3), adapted from Stover and Koepke (14). Table 26. Percentage of cycles.
Number and Spacing of Unsignalized Access Points 39 Pedestrian Safety Bowman et al. (16) developed a predictive model for streets with a raised-curb median. The model relates crash rate to driveway density and indicates that the vehicleâpedestrian crash rate increases with an increase in driveway density. Bicycle Operations Equations 18-41 and 18-44 in the HCM6 (2) are used to determine the effect of motorized vehicle speeds on bicycle LOS along a roadway link (i.e., between traffic signals). 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. Equations 18-46 and 18-47 in the HCM6 (2) can determine the effect of right-side unsignal- ized access density on bicycle LOS along a roadway segment (i.e., considering both the link and its downstream traffic signal). Decreasing the access point density by 10 points per mile improves bicycle LOS by 0.00â0.77 points per mile, with greater improvements occurring when (a) the starting access point density is higher and (b) the starting bicycle LOS for the link is worse. These results assume that the roadway link and the downstream, signalized intersections have identical bicycle LOS scores. Truck Operations Section P in Exhibit 3 of NCHRP Report 825 (17), 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. Additional Information â¢ Chapters 6, 8, 9, and 10 in this guide. â¢ Access Management Manual, Second ed.: Sections 15.3, 15.4, and 20.2. â¢ Access Management Application Guidelines: Chapter 12, Unsignalized Access Spacing. â¢ NCHRP Report 420: Chapter 4, Unsignalized Access Spacing, and Chapter 5, Corner Clearance Criteria. References 1. Texas Transportation Institute; Kittelson & Associates, Inc.; and Purdue University. Predicting the Perfor- mance of Automobile Traffic on Urban Streets. Final report, NCHRP Project 03-79. Transportation Research Board of the National Academies, Washington, D.C., Jan. 2008. 2. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 3. 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. 4. Dixon, K. K., R. D. Layton, M. Butorac, P. Ryus, J. L. Gattis, L. Brown, and D. Huntington. Access Management Application Guidelines. Transportation Research Board, Washington, D.C., 2016. 5. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010.
40 Guide for the Analysis of Multimodal Corridor Access Management 6. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, National Academy Press, Washington, D.C., 1999. 7. 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. 8. 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. 9. National Association of City Transportation Officials. Urban Street Design Guide. National Association of City Transportation Officials, Washington, D.C., Oct. 2013. 10. Hemily, B., and R. D. King. TCRP Synthesis 75: Uses of Higher Capacity Buses in Transit Service. Transportation Research Board of the National Academies, Washington, D.C., 2008. 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. Gan, C.-T., and G. Long. Effects of Inadequate Driveway Corner Clearances on Traffic Operations, Safety, and Capacity. Transportation Research Record: Journal of the Transportation Research Board, No. 1579, 1997, pp. 35â42. 14. Stover, V. G., and F. J. Koepke, Transportation and Land Development, 2nd ed. Institute of Transportation Engineers, Washington, D.C., 2002. 15. Stokes, A., W. A. Sarasua, N. Huynh, K. Brown, J. H. Ogle, A. Mammadrahimli, W. J. Davis, and M. Chowdhury. Safety Analysis of Driveway Characteristics Along Major Urban Arterial Corridors in South Carolina. Presented at 95th Annual Meeting of the Transportation Research Board, Washington, D.C., Jan. 2016. 16. Bowman, B., R. Vecellio, and J. Miao. Vehicle and Pedestrian Accident Models for Median Locations. Journal of Transportation Engineering, Vol. 121, No. 6, American Society of Civil Engineers, Washington, D.C., Nov.âDec. 1994, pp. 531â537. 17. 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.