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21 Description A service road is a local roadway parallel to an arterial roadway whose function is to provide direct access to properties adjacent to the arterial; it may be located in front of or in back of properties, relative to the arterial. A frontage road is a type of service road located between the arterial and the adjacent property (1). Access to the service road may occur at intersections with crossroads that intersect the arterial or, in the case of some one-way frontage roads, via slip ramps between the arterial and frontage road. Tables 11 through 13 follow. Quantitative Analysis Methods Motor Vehicle Operations Chapter 18 in the HCM6 (3) provides methods for estimating the change in arterial travel speed between traffic signals resulting from shifting property access from an arterial to a service road. In a first method, HCM6 Equation 18-3 and Exhibit 18-11 give the reduction in free-flow speed due to access point density (3). 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) 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. C H A P T E R 4 Frontage and Service Roads Source: Photograph provided by the authors.
22 Guide for the Analysis of Multimodal Corridor Access Management Access Management Technique Performance Trends and Documented Performance Relationships Operations Safety Install frontage road to provide access to individual parcels. â â â â â â â â ï ï ï ï ï ï ï Increase distance from service road to arterial along crossroad. â â â ï ï â â â ï ï ï ï Construct service road behind properties abutting the arterial. â â â â ï â â â ï ï ï ï ï Construct bypass road to remove through traffic from arterial. â â â â â â â â ï ï ï ï ï ï Note: â = unchanged performance. Table 11. Multimodal operations and safety performance summary. Table 12. General trends associated with developing frontage and service roads. Mode Operations Safety Motor vehicle free flow and travel speeds increase by up to a few miles per hour between intersections. Where service roads are accessed from crossroads that intersect the arterial at traffic signals, delay to the arterial roadway may increase due to the increased turning movement volumes at the signalized intersection (2, 3). Service roadâcrossroad intersections located too close to crossroadâarterial intersections may be blocked by queued traffic (4). Signalized service roadâcrossroad intersections located close to crossroadâarterial intersections create signal timing challenges (4). The motorized vehicle crash rate on the arterial roadway decreases (4, 5). Turning movements to and from properties occur in a lower-volume, lower-speed environment rather than on the arterial, thereby improving safety (4). Better separation of conflict points when service roadâ crossroad intersections are located farther away from crossroadâarterial intersections (4). Reduced conflict points when access is shifted to a service road behind properties abutting the arterial. All other factors being equal, pedestrian LOS will be better on a service road than along the arterial, due to lower traffic volumes and potentially lower speeds (3). Pedestrian travel times may increase due to less-direct routes (e.g., bowing of frontage roads away from the arterial at crossroads), greater delay at unsignalized crossroadâfrontage road intersections as opposed to using a signalized arterialâcrossroad intersection, or needing two traffic signal cycles to fully cross the widened arterial (where frontage roads are immediately adjacent to the main roadway). Better separation of conflict points when service roadâcrossroad intersections are located farther away from crossroadâarterial intersections (4). Reduced conflict points when access is shifted to a service road behind properties abutting the arterial. Unsignalized service roadâcrossroad intersections may be more challenging to cross than signalized arterialâcrossroad intersections. When bicycle traffic is relocated from the arterial to a service road, bicycle LOS between intersections will improve due to the lower traffic volumes, typically lower heavy vehicle percentages, and potentially lower traffic speeds (3, 6). However, bicycle travel times may increase due to less direct routings and delay at unsignalized crossroadâservice road intersections. One-way service roads may force out-of-direction bicycle travel to access land uses on the opposite side of the arterial. Reduced vehicle speeds along service roads, relative to the arterial, may positively affect bicycle safety (8, 9). Better separation of conflict points when service roadâcrossroad intersections are located farther away from crossroadâarterial intersections (4). Reduced conflict points when access is shifted to a service road behind properties abutting the arterial. Unsignalized service roadâcrossroad intersections may be more challenging to cross than signalized arterialâcrossroad intersections.
Frontage and Service Roads 23 Mode Operations Safety Buses remaining on an arterial where frontage roads are installed will have reduced flexibility for locating bus stops (particularly to serve midblock trip generators). Buses diverting to a service road will experience lower travel speeds, which lowers bus LOS (3, 6). No documented effect beyond that generally observed for motor vehicle traffic. Similar to motor vehicles on the arterial, but with greater benefit for trucks, as they accelerate more slowly to their running speed after stopping or slowing (9). Improved truck LOS due to improved speeds (10). No documented effect beyond that generally observed for motor vehicle traffic. Table 12. (Continued). In a second method, Exhibit 18-13 gives the average through vehicle delay in terms of seconds per vehicle per full, unsignalized access point (3), as shown in Table 13. Delay values in this table 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 percentages. Reduce the delay values by 50% if one turning movement is provided with an appropriately dimensioned turn lane or a 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 Chapter 9 for additional information specific to turn lanes. Chapter 19 in the HCM6 can be used to determine the change in delay at signalized inter- sections along the arterial resulting from increased turning movement volumes, as well as delay at signalized crossroadâservice road intersections (8). Chapters 20 to 22 in the HCM6, which address different types of unsignalized intersections, can be used to evaluate the operation of unsignalized crossroadâservice road intersections (3). Chapters 19 and 20 can be used to determine the 95th-percentile queue length on crossroads at arterialâcrossroad intersections (3). This distance is a factor in determining the minimum separation between arterialâcrossroad and arterialâservice road intersections. Motor Vehicle Safety Frontage and service roads segregate through local access traffic and thereby reduce the fre- quency and severity of conflicts on the arterial road (4). Because conflict points are relocated from the main roadway to the service road, there may not be any change in the total number of conflict points (depending on the turning movements allowed at each driveway before and after Midsegment Volume (vehicles per hour per lane) Through Vehicle Delay (seconds per vehicle per full, unsignalized 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 13. Average through vehicle delay.
24 Guide for the Analysis of Multimodal Corridor Access Management the development of service roads). However, because service roads are lower-volume environ- ments and (frequently) lower-speed environments, the number and severity of crashes would be expected to be lower, relative to the situation without service roads. Drawing from a review of a number of crash studies, NCHRP Report 420 (4) indicated that each additional access point per mile increases 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 a road 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): ⢠Reducing driveways from 48 to 26â48 per mile: 0.71 ⢠Reducing driveways from 26â48 to 10â24 per mile: 0.69 ⢠Reducing driveways from 10â24 to less than 10 per mile: 0.75 Pedestrian Operations Equations 18-32 and 18-35 in the HCM6 (3) 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 (6). Chapter 19 in the HCM6 can be used to determine the changes in pedestrian crossing delay and pedestrian intersection LOS at signalized intersections that result from the development of frontage roads. Chapter 20 in the HCM6 can be used to estimate pedestrian delay at two-way stop-controlled service roadâcrossroad intersections (3). Bicycle Operations Equations 18-41 and 18-44 (3) are used to 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. Equations 18-46 and 18-47 (3) can determine the effect of unsignalized access spacing on bicycle LOS. 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 points while decreasing the access point density by 20 points per mile improves bicycle LOS by 0.28 points. These results assume that heavy vehicles make up 5% of the traffic volume and that roadway links and signalized inter- sections are weighted the same when calculating overall bicycle LOS. Chapter 19 in the HCM6 (3) can be used to determine the changes in bicycle delay and bicycle intersection LOS at signalized intersections that result from the development of frontage roads. Bus Operations Chapter 18 in the HCM6 (3) can be used to estimate the change in average bus speeds resulting from reduced traffic volumes, changes in routing, or both. This information in turn can be used to estimate the change in bus LOS for the segment. At 30-minute bus headways with seated loads and reliable service, a 1-mph increase in average bus speed produces a
Frontage and Service Roads 25 0.08â0.12 improvement in the bus LOS score, with 0.75 points representing the range covered by one LOS letter (3, 10). Truck Operations Section P in Exhibit 3 of NCHRP Report 825 (11), 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 ⢠Chapter 12 in this guide. ⢠Access Management Manual, Second ed.: Sections 20.4.10 and 20.4.11. ⢠Access Management Application Guidelines: Section 19.3.1. ⢠NCHRP Report 420: Chapter 10, Frontage Roads. 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. 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. 3. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed. Transportation Research Board, Washington, D.C., 2016. 4. Gluck, J., H. S. Levinson, and V. Stover. NCHRP Report 420: Impacts of Access Management Techniques. Transportation Research Board, Washington, D.C., 1999. 5. Highway Safety Manual, 1st ed. American Association of State Highway and Transportation Officials, Washington, D.C., 2010. 6. 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. 7. 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. 8. National Association of City Transportation Officials. Urban Street Design Guide. National Association of City Transportation Officials, Washington, D.C., Oct. 2013. 9. 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. 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. 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. 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.
