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NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 41 APPENDIX C. SPEED LIMIT SETTING APPROACHES A well-developed approach to setting speed limits should consider the factors that affect operating speeds. Several factors, including the posted speed limit, can influence a driverâs speed choice. Table 5 provides a list of factors that are believed to affect a driverâs speed choices; however, the exact relationships may not be clear or may not have been conclusively proven. Adding to the challenge of quantifying the relationship is the interaction between these factors and the overall visual scene for the driver. The âlook and feelâ of the road can communicate an appropriate speed to a driver. Of course, the driver must be willing to accept that message. Appendix A (urban/suburban streets) and Appendix B (high-speed roads) include findings on the relationships among safety, operating speed, and roadway characteristics including posted speed limit. Table 5. Example of factors that could influence a driverâs speed choice. Categories Factors Crash/Incident Number and/or type of crashes, nearby crash, vehicle on shoulder or roadside Enforcement Presence of enforcement personnel, suspected enforcement levels, tolerance Environment Weather, light (dawn, day, dusk, night), visibility Function Functional classification, overall visual scene for driver Human Driver age, driver skill, personality of driver, emotional and/or physical condition of driver, familiarity of driver with roadway, influence of alcohol or drugs, number of passengers, type of passengers Other Work zone, school zone Pavement Pavement type and condition, pavement roughness, surface condition (e.g., wet, ice, etc.) Road Number of lanes, lane width, median type, roadside clearance, roadway alignment (horizontal and vertical), on-street parking, on-road bicyclist facilities, available sight distance, shoulder presence/width, sidewalk characteristics Roadside Land use, signal spacing, driveway/intersection (access) density, development type, zoning Traffic Vehicle volume, percent heavy trucks, speed of other vehicles, pedestrians, children, bicyclists (presence and location) Traffic control devices Signs (including posted speed limit), signals, pavement markings Trip Time of day, purpose of trip, urgency of trip, length of trip Vehicle Type of vehicle, condition of vehicle, vehicle size A limited-access road (freeway) with multiple lanes and wide roadside clear zone communicates the appropriateness of high operating speeds, while a residential street with on- street parking, multiple driveways, and pedestrian activity communicates the need for low speeds. The posted speed limit needs to be in agreement with the design of the road if desired operating speeds are to be achieved. When the design of the roadâin terms to how it visually looks to a driverâresults in implying that a higher operating speed is reasonable, engineering treatments may be needed to adjust the message being communicated to the driver. Forbes et al. (166) stated that the methodologies for setting speed limits typically are designed to result in recommended speed limits that: ï· Are related to crash risk. ï· Provide a reasonable basis for enforcement.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 42 ï· Are fair in the context of traffic law. ï· Are accepted as reasonable by a majority of road users. RECENT KEY PUBLICATIONS CALLING FOR CHANGES TO HOW SPEED LIMITS ARE SET Recently, the speed limit debate has increased with two publications. In March 2017, NACTO released a policy statement (6). One of the action items in that statement would âpermit local control of city speed limits.â NACTO recommends that âstate rules or laws that set speed limits at the 85th percentile speed should be repealed.â In July 2017, NTSB published a report on speeding (Reducing Speeding-Related Crashes Involving Passenger Vehicles) (7). That document included several recommendations for reducing speed-related crashes including two recommendations directed to FHWA for changes to the MUTCD (8, pp. 57): ï· Revise Section 2B.13 of the Manual on Uniform Traffic Control Devices so that the factors currently listed as optional for all engineering studies are required, require that an expert system such as USLIMITS2 be used as a validation tool, and remove the guidance that speed limits in speed zones should be within 5 mph of the 85th percentile speed (H-17-27). ï· Revise Section 2B.13 of the Manual on Uniform Traffic Control Devices to, at a minimum, incorporate the safe system approach for urban roads to strengthen protection for vulnerable road users (H-17-28). OVERVIEW ON APPROACHES TO SETTING SPEED LIMITS Several approaches are used to set speed limits, and Table 6 summarized those approaches. Advantages and disadvantages are provided in Table 7. Two approaches to setting speed limits that have gained attention in recent years include the citywide default speed limit and the greater use of slow zones that include the implementation of a lower speed limit for a region. When adjusting or modifying a statutory speed limit for a roadway segment, the most common approach for setting speed limit is the engineering operating speed approach. Engineering (Operating Speed) The operating speed (engineering) approach relies on the 85th percentile speed with adjustments used to account for existing roadway geometry or crash experience. It is the most common method used in the United States. Additional details about this approach are discussed in later sections of this appendix (see MUTCD and Online Review of State Guidance sections on setting speed limits). Engineering (Road Risk) With a road risk engineering approach, the speed limit is determined by the risks associated with the physical design of the road and the expected traffic conditions. Essentially, the approach sets the speed limit according to the function or classification of the road and then adjusts the speed limit based on the relative risk introduced by various road and roadside design features. Discussion on two examples of this approach follows.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 43 The road risk (engineering) approach uses the risks associated with the physical design of the road and the expected traffic conditions to set the speed limit. Typically, the functional classification of the road sets the initial value, which is then adjusted based on the relative risks introduced by road or roadside features. While existing operating speeds are not used to set the speed limit, users are encouraged to compare the operating speeds to the proposed posted speed limit and, when the operating speed is higher, to consider engineering techniques that could be used to lower vehicle operating speeds. Table 6. Approaches to setting speed limits. Approach Description Examples Engineering (Operating Speed) A two-step process where a base speed limit is set according to the 85th percentile speed, the design speed for the road, or other criterion. This base speed limit is adjusted according to traffic and infrastructure conditions such as pedestrian use, median presence, etc. Most states and cities in the United States Engineering (Road Risk) Speed limit is determined by the risks associated with the physical design of the road and the expected traffic conditions. Essentially, the approach sets the speed limit according to the function or classification of the road and then adjusts the speed limit based on the relative risk introduced by various road and roadside design features. Canada, New Zealand Expert System Speed limits are set by a computer program that uses knowledge and inference procedures that simulate the judgment and behavior of speed limit experts. The system contains a set of rules for applying the knowledge to each situation. United States (e.g., USLIMITS2), Australia Optimization Speed limits are set to minimize the total societal costs of transport. Travel time, vehicle operating costs, road crashes, traffic noise, and air pollution are considered in the determination of optimal speed limits. Approach is a concept that has not yet been adopted by any road authority Injury Minimization/ Safe System Speed limits are set according to the crash types that are likely to occur, the impact forces that result, and the human bodyâs tolerance to withstand those forces. Sweden, Netherlands, Australia Citywide or Default Speed limits are set based on region (e.g., a city) or type of street (i.e., neighborhood streets) within a region. Boston, Massachusetts; New York City, New York; Seattle, Washington; Portland, Oregon Slow Zones Small area consisting primarily of local streets with signs and markings that alert drivers to the reduced speed limit. New York City, New York; London, England Source: Table created using (a) material provided in FHWA-SA-12-004 (166), pages 9â24, especially for engineering, expert system, optimization, and injury minimization; and (b) material generated by authors of this report.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 44 Table 7. Advantages and disadvantages for approaches to set speed limits. Approach Data Required Advantages Disadvantages Engineering (Operating Speed) The existing speed profile as well as data on accesses, pedestrian/bicycle traffic, curbside parking, safety performance, etc. Using the 85th percentile speed ensures that the speed limit does not place an undue burden on enforcement and provides residents and businesses with a valid indication of actual travel speeds. Drivers may not be adequate judges of the externalities of their actions and may not be able to self-select the most appropriate travel speed. Speed limits are often set lower than the 85th percentile speed. Engineering (Road Risk) Functional classification of the road, setting (urban/ rural), surrounding land uses, access, design features of the road. The speed limit and the function of the road are aligned. The function of the road also dictates many of the design elements of the road, so this method aligns the speed limits with the design of the road. The road risk methods may result in speed limits that are well below the 85th percentile speeds, resulting in an increased burden on enforcement if remedial measures are not employed (i.e., traffic calming, etc.). Expert System Data needs depend on the system, but generally expert systems require the same data as used in the engineering approaches. A systematic and consistent method of examining and weighing factors other than vehicle operating speeds in determining an appropriate speed limit. It is reproducible and provides consistency in setting speed limits. Practitioners may need to rely on output from the expert system without applying a critical review of the results. Injury Minimization/ Safe System Crash types and patterns for different road types, and survivability rates for different operating speeds. There is a sound scientific link between speed limits and serious crash prevention. Places a high priority on road safety. This method is based solely on a road safety premise and may not be accepted as appropriate in some jurisdictions. Citywide or Default City must have authority to set speed limits within its region. Consistency within a region. Fewer signs since signs may only be needed at entrances to region. Removes connection between driverâs interpretation of appropriate speed for the facility. Slow Zones Applications for a slow zone need to contain the streets to include along with letters of support from key stakeholders. Focuses on the needs of all users within an area. Limited to certain roadway types. Could be labor intensive for a city to implement. Roadway changes may be needed for the revised speed limit to be effective. Source: Table created using (a) material provided in FHWA-SA-12-004 (166), pages 9â24, especially for engineering, expert system, optimization, and injury minimization; and (b) material generated by authors of this report.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 45 New Zealand Speed Limits The objective from New Zealandâs speed limit policy (dated 2003) was âto balance the interests of mobility and safety by ensuring speed limits are safe, appropriate, and credible for the level of roadside development and the category of road for which they are setâ (13). The selection of the proposed posted speed limit is based on a review of the roadside development and the general road information. Rating units are assigned to each 100-m section within the study area. The rating units consider frontage and side road development characteristics, pedestrians, cyclists, parking, geometry, traffic control, and development type. The average rating for the section is then used within one of three flow charts (rural, in between, or urban) to determine the speed limit. A flow chart is used to show the connection between the average rating and the speed limit, for example, in a rural area, an average rating of 6 or more and less than 11 would result in a 43.5 mph (70 km/h) speed limit. A November 2016 publication replaced the guidance discuss in the previous paragraph (167). The New Zealand Speed Management Guide First Edition outlines a speed management framework that encompasses elements of the Safe System approach to reduce the risk of death and serious injury, while supporting overall economic productivity. Canadian Guidelines for Establishing Posted Speed Limits The Transportation Association of Canada has a publication that provides an evaluation tool to assess appropriate posted speed limits based on the classification, function, and physical characteristics of a roadway. It is called the Canadian Guidelines for Establishing Posted Speed Limits (168). The methodology is based on risks associated with engineering factors with the lower recommended posted speed limits for higher risk. The guidelines were developed through a review of current domestic and international practices, technical documentation, and testing. An automated spreadsheet is provided to facilitate the evaluation of posted speed limits. Elements considered are geometry, roadside, classification, land use, access and intersection density, and vulnerable road users. The 85th percentile speed is used as a check, not as the determining factor. Expert System Example With an expert system, speed limits are identified with a computer program that applies a set of rules for each situation. An expert system currently in use in the United States is USLIMITS2. USLIMITS2 USLIMITS is a web-based tool. The FHWA website (169) describes it as: a web-based tool designed to help practitioners set reasonable, safe, and consistent speed limits for specific segments of roads. USLIMITS is applicable to all types of roads ranging from rural local roads and residential streets to urban freeways. User friendly, logical, and objective, USLIMITS2 is of particular benefit to local communities and agencies without ready access to engineers experienced in conducting speed studies for setting appropriate speed limits. For experienced engineers, USLIMITS2 can provide an objective second opinion and increase confidence in speed limit setting decisions.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 46 Links available on the FHWA website provide access to the software to run USLIMTS2 along with links to the following resources: ï· User Guide (15). ï· Decision Rules (170). ï· NCHRP 3-67 Project Report (11). ï· Frequently Asked Questions. The core of USLIMITS2 is a set of decision rules developed by two selected groups of experts. One of the flow charts within the decision rules shows that the three roadway types included are: ï· Limited-access freeway. ï· Road section in undeveloped areas. ï· Road section in developed areas, which is subdivided into: o Residential subdivision/neighborhood street (posted speed limits generally range from 25 mph to 35 mph). o Residential collector street (posted speed limits generally range from 25 mph to 45 mph). o Commercial street (posted speed limits generally range from 25 mph to 45 mph). o Street serving large complexes (posted speed limits generally range from 35 mph to 50 mph). The needed input variables are listed in Table 8. For the crash statistics, the user is to provide (a) total number of crashes in the section, (b) total number of injury and fatal crashes in the section, and (c) the average AADT for the study period. The calculated rate of total crashes and rate of injury and fatal crashes are compared to average rates. The average rates are determined based on user-provided data for similar sections (i.e., number of lanes, median type, traffic volumes, area type, etc.). If the user does not provide the average rates, USLIMTS2 uses default values from the Highway Safety Information System (HSIS). Per the user manual (11), the average crash and injury rates were calculated using the latest 3 years of data available from California (2000â2002), Illinois (2001â2003), Maine (2002â2004), Minnesota (2002â2004), North Carolina (2001â2003), Ohio (2002â2004), Utah (1998â2000), and Washington (2002â 2004). The 2017 USLIMITS2 user manual (15) has updated crash rates and injury rates. They were calculated using the latest 3 years of data that were available at the time of the update: California (2009â2011), Minnesota (2010â2012), North Carolina (2011â2013), Ohio (2010â 2012), and Washington (2010â2012). Several flow charts are available to illustrate the decision process within USLIMITS2. An example flow chart for speed limit calculations without crash data for a roadway section in developed areas is shown in Figure 3. For the âextent of pedestrian/bicyclist activityâ variable, the users are asked to select between âhighâ and ânot high.â Examples of areas with high pedestrian and bicycle activity include the following: ï· Residential developments with four or more housing units per acre interspersed with multifamily dwellings. ï· Hotels located with 1/2 mi of other attractions such as retail stores, recreation areas, or senior centers. ï· Downtown or central business district (CBD) areas. ï· Areas with paved sidewalks, marked crosswalks, and pedestrian signals.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 47 USLIMTS provides suggested minimum section lengths (see Table 9). The program provides warnings when the 85th percentile speed exceeds 77 mph for a limited-access freeway, 67 mph for a non-limited-access road in an undeveloped area, and 52 mph for a non-limited- access road in a developed area. Source: USLIMITS decision rules flow chart (170), page K-23. Transportation Research Board. 2006. Expert System for Recommending Speed Limits in Speed Zones: NCHRP 03-67. Adapted and Reproduced with permission from the National Academy of Sciences, Washington, DC. Figure 3. USLIMITS2 flow chart for speed limit calculation without crash data for roadway section in developed areas.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 48 Table 8. Input variables required for USLIMITS2. Input Variables Limited-Access Freeway Road Section in Undeveloped Area Road Section in Developed Area Operating speed (85th and 50th percentile speed) Yes Yes Yes Section length Yes Yes Yes AADT Yes Yes Yes Presence/absence of adverse alignment Yes Yes Yes Current statutory limit for this type of road Yes Yes Yes Terrain Yes No No Is this section transitioning to a non- limited-access highway? Yes No No Is this section transitioning to a road section in a developed area? No Yes No Number of interchanges within this section Yes No No Crash statisticsa Yes Yes Yes Roadside rating No Yes No Divided/undivided section No Yes No Number of through lanes No Yes Yes Whether it is a one-way street No No Yes Area type No No Yes Number of driveways within the section No No Yes Number of traffic signals within the section No No Yes Presence/usage of on-street parking No No Yes Extent of pedestrian/bicyclist activity No No Yes a At least 3 years of crash data recommended; if less than 1 year of data, USLIMITS2 suggests that additional data should be collected and the process repeated. Source: Table created from list provided in USLIMITS user guide (11), page L-17. Table 9. USLIMITS2 suggested minimum length per speed limit. Speed Limit (mph) Minimum Length (mi) Speed Limit (mph) Minimum Length (mi) 30 35 40 45 50 0.30 0.35 0.40 0.45 0.50 55 60 65 70 75 0.55 1.20 3.00 6.20 6.20 Source: USLIMITS user guide (11), Table L.2, page L-34. Citywide or Default Speed Limits Default speed limit is the speed limit set by government action, typically a state or city for an area. The default speed limit can depend on the type of the road and its location (e.g., rural or urban) and the vehicle type and its properties. Several U.S. cities have recently campaigned to be able to set lower citywide default speed limits. For example, Boston, Massachusetts; New York City, New York; and Seattle,
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 49 Washington, now have the ability to set a 25-mph speed limit citywide. Portland, Oregon, has the authority to set residential streets at 20 mph. Other countries are also implementing citywide speed limits. In Bristol, UK, a 20-mph speed limit policy was implemented between 2010 and 2015 in phases within seven areas of the city. Researchers (171) used crash data from 2008 to 2016 to investigate the change in crashes. The 20-mph speed limit intervention was associated with a city-level reduction of fatal injuries of around 63 percent after controlling for trends over time and areas. There was also a general trend of reduction of the total number of injuries at the city level and on 20-mph roads. The authors hypothesized that a citywide speed limit approach may encourage a general behavior change in drivers that, in turn, may contribute to reducing injuries across the city. Hu and Cicchino (172) investigated the effects of a speed limit reduction on speeds in Boston. On January 9, 2017, the default speed limit on City of Boston streets was reduced from 30 mph to 25 mph. The researchers collected vehicle speeds at 50 sites in Boston where the speed limit was lowered from 30 mph to 25 mph. They also collected vehicle speeds at 50 control sites in Providence, Rhode Island, where the speed limit remained at 25 mph before and after the speed limit change in Boston. Table 10 shows the findings from the speed studies. The researchers used a log-linear regression model to estimate the change in vehicle speeds associated with the speed limit reduction and separate logistic regression models to estimate changes in the odds of vehicles exceeding 25 mph, 30 mph, and 35 mph. They found a 0.3 percent reduction in mean speeds (p=0.065) and reductions of 2.9 percent, 8.5 percent, and 29.3 percent in the odds of vehicles exceeding 25 mph, 30 mph, and 35 mph, respectively. The authors concluded that while the average and 85th percentile speeds did not change meaningfully, the reductions in the proportions of vehicles traveling at higher speeds have important implications for nonmotorists.Â Table 10. Speeds and proportion of vehicles exceeding a speed from Boston citywide speed limit change study. Period Boston (50 study sites, speed limit changed from 30 mph to 25 mph) Providence (50 control sites, speed limits at 25 mph) Measure Mean Speed (mph) 85th %-ile Speed (mph) % above 25 mph % above 30 mph % above 35 mph Mean Speed (mph) 85th %-ile Speed (mph) % above 25 mph % above 30 mph % above 35 mph Before 24.8 31.0 47.9 18.2 4.9 24.8 31.0 45.9 15.9 3.5 After 24.8 31.0 46.9 18.1 3.8 24.9 31.0 46.2 17.5 4.1 Before-to-after change 0% 0% â2.1% â0.5% â22.4% 0.4% 0% 0.7% 10.1% 17.1% Source: Hu W., & Cicchino J. B. (2019) Lowering the Speed Limit From 30 mph to 25 mph in Boston: Effects on Vehicle Speeds. Injury Prevention, 0, 1-4. (172), Table 1, page 7. On October 24, 2016, Oregon provided the City of Portland approval to begin use of its proposed experimental alternative speed zone investigation method. The alternative method (12) is to be used on streets that are under the jurisdiction of the City of Portland. The justification for the alternative method is safety, with the principle factors to determine risk being the speed of adjacent motor vehicles and proximity of those vehicles to the more vulnerable road users, namely pedestrians and cyclists. The determination of the recommended posted speed limit for a corridor is the lowest recommended speed for each road userâpedestrian, bicyclist, and auto. Portland notes on its website (173) that speeds âmust account for people traveling in different ways: walking, driving, using mobility devices, biking, skateboarding, etc.â and that âit
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 50 is important to consider people traveling outside of motor vehicles because they are not protected from the impact of crashes.â Portland provides the following four methods that the city can use to request speed limit changes: ï· Alternative method. For use on non-arterial streets with speed limits above 25 mph. Uses a streamlined request process that places greater emphasis on vulnerable users and the risk of a future crash relative to the traditional method. ï· Traditional method. Required on arterial streets except on sections eligible for business district statutory speed limits. Uses multiple factors to determine speed limits, including 85th percentile speeds, crash history, roadside culture, traffic volumes, roadway alignment, width, and surface. ï· Statutory method. For streets with a speed limit specified by law. ï· Special clauses. Allows for 5 mph below statutory speed limits on certain streets such as low-traffic neighborhood greenways and certain residential streets. Slow Zones Slow zones are corridors or regions with a lower speed limit than surrounding areas. An example of a slow zone program is the Neighborhood Slow Zones program implemented by New York City (174). The goals of the Neighborhood Slow Zones program are to lower the incidence and severity of crashes and to enhance quality of life by reducing cut-through traffic and traffic noise in residential neighborhoods. Within the slow zone area, speed limits are reduced from 25 mph to 20 mph, and roadway geometric treatmentsâsuch as speed bumps or other traffic calming treatmentsâare added with the intention of changing driver behavior. Gateway signs and markings are used at intersections to alert drivers to the reduced speed limit. Neighborhood Slow Zones are typically established in small, self-contained areas that consist primarily of local streets where the streets within the zones can be self-enforcing due to the roadway characteristics. They are implemented in areas with low-traffic volumes and minimal through traffic, where reducing the speed limit will not cause traffic congestion. New York City has reported that areas where Neighborhood Slow Zones have been implemented experienced a 10â15 percent decrease in speeds, 14 percent reduction in crashes with injuries, and 31 percent reduction in vehicle injuries (174). MANUAL ON UNIFORM TRAFFIC CONTROL DEVICES The MUTCD (8) provides information on setting non-statutory speed limits. Within Section 2B.13 of the MUTCD are several paragraphs related to the selection of the posted speed limit value, including Paragraph 1 (standard), Paragraph 12 (guidance), and Paragraph 16 (option). Other paragraphs within Section 2B.13 focus on statutory speed limits, the need for engineering studies, requiring limits to be multiples of 5 mph, placement of signs, use of warning signs with speed limit signs, where to conduct speed studies, special speed limits, changeable message signs, and school zones. Relative to setting speed limits, the key paragraphs in Section 2B.13 are: 01 Speed zones (other than statutory speed limits) shall only be established on the basis of an engineering study that has been performed in accordance with traffic engineering practices. The engineering study shall include an analysis of the current speed distribution of free-flowing vehicles.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 51 12 When a speed limit within a speed zone is posted, it should be within 5 mph of the 85th-percentile speed of free-flowing traffic. 16 Other factors that may be considered when establishing or reevaluating speed limits are the following: o Road characteristics, shoulder condition, grade, alignment, and sight distance, o The pace, o Roadside development and environment, o Parking practices and pedestrian activity, and o Reported crash experience for at least a 12-month period. To address the NTSB recommendations (7), the NCUTCD Regulatory and Warning Signs (RW) Technical Committee established a task force to explore current and potential approaches to the setting of posted speed limits. The task force started the process by creating and distributing the survey in the spring of 2018 to gather an understanding of how practitioners actually applied current practices in setting speed limits and queried their opinions on the topic. The availability of the web-based survey was announced to members of the following groups: NCUTCD, Institute of Transportation Engineers, AASHTO Committee on Traffic Engineering, American Public Works Association, National Association of County Engineers, American Society of Civil Engineers, NACTO, Association of Pedestrian and Bicycle Professionals, and TRB. The survey included 13 questions, with the initial questions focusing on individualsâ backgrounds and the remaining questions focusing on how they conduct speed studies or what they think a speed study should consider. A total of 740 participants completed the survey. Details on the survey and key findings are documented elsewhere (175, 176). The following general revisions to the MUTCD were suggested based on the survey and discussions within the committee and elsewhere: ï· Change the MUTCD to reinforce the stated understanding that other factors have a role in setting speed limits (in addition to 85th percentile). Refine the factors in Paragraph 16 and group the paragraphs that speak to setting of speed limits. ï· Retain references to 85th percentile as a factor that should be considered, particularly for freeways, expressways, and rural areas. ï· Keep the MUTCD broad. While it could be reorganized to better present the material for setting speed limits (with minor reorganization), it should not be expanded into greater detail. The detail will ideally be provided from national research, state / local procedures and promoted by FHWA. ï· Not include references to specific processes, such as USLIMITS2, but promote this level of detail in state/local procedures and investigate more deeply the reasons why after more than 20 years, only a small fraction of practitioners utilize this expert system. Details on the specific recommendations forwarded to FHWA are available on the NCUTCD website (177). The survey along with discussions within NCUTCD generated the following questions (and suggested direction) that need additional consideration (175): ï· To what extent should the MUTCD define procedures/criteria for posted speed limit engineering studies? o There is support on both sides of the question as to whether the MUTCD should be focused on simply traffic control device criteria (e.g., sign size, color, or shape)
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 52 or both traffic control device criteria and the criteria for setting of speed limits. After review of the survey results and discussion with the NCUTCD Council, the direction was to keep the MUTCD material regarding setting speed limits broad, allowing states/local agencies to define the procedures in more detail. ï· Given the implicit understanding of what 85th percentile means, is there a need to better define the five items in Section 2B.13, Paragraph 16 to build a more uniform level of understanding (e.g., what defines crash experience comparable to our understanding of the 85th percentile)? o Greater definition should be left to national research and state/local procedures rather than expanding upon them in the MUTCD. ï· Why are bicyclists not noted in Paragraph 16? Should any criteria be added to Paragraph 16? o In reviewing the MUTCD history, this list was added in 1971. o It is reasonable to add several factors to Paragraph 16, such as road context, bicyclists, lane width, median type, and/or number of driveways. ï· What is the balance between âanalysis of the current speed distribution of free- flowing vehiclesâ (MUTCD standard Paragraph 1) to other criteria (MUTCD Paragraph 12) as part of an engineering study? How might this affect Paragraph 12? o This should be left to guidelines, not the MUTCD. ï· Is a specific reference to USLIMITS2 appropriate? o Given the survey finding that 84 percent of the respondents had not utilized USLIMITS2, the question as to âwhyâ should be answered before change to the MUTCD is considered. Adding USLIMITS2 would substantially further the MUTCD role of defining the process or procedure of setting speed limits. This level of detail would be inconsistent with the MUTCD establishing broad criteria of setting speed limits and could impact state/local agencies who have detailed procedures. ï· Should the rounding approach to speed data be defined? o This is a detail of setting a speed limit that would not be appropriate for the MUTCD. It should be part of state/local agency policy documents. ï· What will enforcement and/or the judicial system accept if not the 85th percentile (Paragraph 12)? Could speed limits for high crash corridors be set below the 85th percentile, and is this an MUTCD role or a state/local role in defining the speed limit process? o This should be left to guideline documents and national research rather than the MUTCD. ï· Given the commonality of responses to target speed for various facility types from the survey, should a reference be provided that would guide practitioners to further study when to set speeds above/below certain levels nationally (for example, the 50th percentile response levels of the survey for each facility type)? o This is a detail of setting speed limits and would be better in guidelines (or statutory change/requirements) rather than the MUTCD. ï· A criterion suggested for setting speed limits that is relatively new is âcontextâ location.â Some may consider âroad characteristicsâ or âenvironmentââterms currently in the MUTCDâto be similar in concept. NCHRP Research Report 855 (3) recommends an expanded functional classification system with five roadway types
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 53 (freeways, principal arterial, minor arterial, collector, and local) and five contexts (rural, rural town, suburban, urban, and urban core). These contexts âhave been determined to not only represent unique land use environments, but also identify distinctions that require wholly different geometric design practices in terms of desired operating speeds, mobility/access demands and user groups.â Should the MUTCD recognize these different roadway type/context combinations, especially if different speed limit setting practices are suggested for the different roadway type/context combinations? Table 11 shows the suggested target speeds from NCHRP Research Report 855 (3), and Table 12 summarizes the primary factors associated with each roadway context. o This is a detail of setting speed limits and would be better as a subject of guidelines (or statutory change/requirements) rather than the MUTCD. Table 11. NCHRP Research Report 855 suggested target speed for context/roadway. Context Roadway Rural Rural Town Suburban Urban Urban Core Freeways Not addressed in NCHRP Research Report 855 since âdesigns are based on federally developed standards with little flexibility.â Assumed to be high. Principal Arterial High Low/Med Med/High Low/Med Low Minor Arterial High Low/Med Med Low/Med Low Collector Med Low Med Low Low Local Med Low Low Low Low Note: Suggested target speeds: low (<30 mph), med (30 to 45 mph), high (>45 mph). Source: Adapted from Source: Transportation Research Board. 2018. An Expanded Functional Classification System for Highways and Streets. HTTPS://DOI.ORG/10.17226/24775. Reproduced with permission from the National Academy of Sciences (3), Figure 25, page 37.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 54 Table 12. Characteristics of roadway contexts. Context Density Land Use Setback Rural Lowest (few houses or other structures) Agricultural, natural resource preservation, and outdoor recreation uses with some isolated residential and commercial uses Usually large setbacks Rural Town Low to medium (single- family houses and other single-purpose structures) Primarily commercial uses along a main street (some adjacent single-family residential uses) On-street parking and sidewalks with predominately small setbacks Suburban Low to medium (single- and multifamily structures and multistory commercial) Mixed residential neighborhood and commercial clusters (including town centers, commercial corridors, big-box commercial, and light industrial uses) Varied setbacks with some sidewalks and mostly off-street parking Urban High (multistory, low-rise structures with designated off-street parking) Mixed residential and commercial uses, with some intuitional and industrial uses and prominent destinations On-street parking and sidewalks with mixed setbacks Urban Core Highest (multistory and high-rise structures) Mixed commercial, residential, and institutional uses within and among predominately high-rise structures Small setbacks with sidewalks and pedestrian plazas Source: Based on Source: Transportation Research Board. 2018. An Expanded Functional Classification System for Highways and Streets. HTTPS://DOI.ORG/10.17226/24775. Reproduced with permission from the National Academy of Sciences (3), Table 1, page 10. POSTED SPEED LIMIT COMPARED TO OPERATING SPEED While the MUTCD (8) recommends setting the posted speed limits near the 85th percentile speed, and traffic engineers say that agencies are using the 85th percentile speed to set speed limits, in reality the speed limit is often set much lower (178). At these locations, the 85th percentile operating speeds exceed the posted speed limits, and in many cases, the 50th percentile operating speed is either near or exceeds that posted speed limit as well (48). City Streets Using Donated Spot Speed Studies As part of this NCHRP project, the research team collected operating speed data for several city streets; however, all collected speed data were concentrated in one Texas city. Members of the research team participated in the volunteer NCUTCD Task Force on speed limits, including making presentations on the findings. During some of those presentations, requests for donations of spot speed studies were made. In addition, the chair of the NCUTCD Task Force sent emails requesting the donation of spot speed studies. Several communities provided spot speed studies, most of which were collected using road tubes where the speed data are binned in 5-mph increments. Table 13 lists the number of available speed studies by roadway type and state. Using data collected as a part of this project along with the data donated by several communities, graphs of operating speed versus posted speed limit were developed. Because this project created a database of speeds for the City of Austin, Texas, many of the speed studies are
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 55 from Texas, but over half are from states other than Texas (see Table 13). The speed studies were assigned as either an arterial, a collector, or a local street for this comparison, and only speed studies on city streets (i.e., urban or suburban roadway context) were included. Figure 4 shows the plot for urban arterial, Figure 5 for urban collectors, Figure 6 for urban local streets, and Figure 7 for all 875 urban street speed studies. Figure 8 shows the distribution of average operating speed by posted speed limit using box plots for all 875 urban street speed studies. Table 13. Number of speed studies by roadway type and state. State Arterial Collector Local Total AR 5 3 0 8 AZ 18 0 0Â 18 CA 0 116 0 116 DE 4 0 0Â 4 FL 0 5 0 5 IL 0 93 62 155 MA 7 1 0 8 MD 0 6 0 6 MI 1 14 2 17 MO 6 3 2 11 NH 10 48 8 66 OR 3 7 2 12 TN 5 3 3 11 TX 133 182 116 431 VA 2 5 0 7 Total 194 486 195 875
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 56 Figure 4. Operating speed by posted speed limit for urban arterials. Figure 5. Operating speed by posted speed limit for urban collectors.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 57 Figure 6. Operating speed by posted speed limit for urban locals. Figure 7. Operating speed by posted speed limit for city streets.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 58 Figure 8. Using box plots to illustrate average operating speed by posted speed limit for city streets. The four graphs clearly show a large range of values for both the average speed and the 85th percentile speed within each posted speed limit. For example, the 39 arterials with a 40-mph posted speed limit in Figure 4 show an average speed that ranges from 31 to 46 mph and an 85th percentile speed that ranges from 39 to 52 mph. These ranges indicate that other factors are influencing driversâ speeds in addition to posted speed limit. A general observation from the graphs is that the political process could also be influencing the setting of a speed limit below the 85th percentile at some of the sites. Overall, trendlines can provide an appreciation of whether the posted speed limit is more similar to the average speed or the 85th percentile speed. As illustrated in each graph, the trendline for the average speed is closer to the posted speed limit than the trendline for the 85th percentile speed. As shown in Figure 7 for all city streets, the average speed is about equal to the posted speed limit for posted speed limits below 40 mph and then within about 1 mph for posted speed limits at 40 or above. For this set of city street speed data, the 85th percentile speed could be generalized as being the posted speed limit plus 4 mph. Freeways and Two-Lane Highways Using Naturalistic Driving Study Data Savolainen et al. (179), using data from the Second Strategic Highway Research Program (SHRP2) Naturalistic Driving Study (NDS), identified driver speed and posted speed limit relationships for freeways and rural two-lane highways. The freeway dataset included a total of 4,375 driving traces for 1,975 unique drivers at four different posted speed limits ranging from 55 mph to 70 mph. The authors had the ability to also compare results when traffic was flowing at level of service (LOS) A and found the following: ï· 55 mph posted speed limit and LOS A: about 58 mph average speed. ï· 60 mph posted speed limit and LOS A: about 63 mph average speed.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 59 ï· 65 mph posted speed limit and LOS A: about 69 mph average speed. ï· 70 mph posted speed limit and LOS A: about 73 mph average speed. So, during conditions when traffic is light, the average freeway speed is about 3 mph higher than the posted speed limit. This finding is in contrast to the findings for city streets, where the average speed is about equal to the posted speed limit. The authors developed mixed-effect linear regression models and found that mean speeds are largely affected by the level of traffic congestion. Because they had information on driver age, the authors were able to identify that speeds were higher among younger and middle-aged drivers compared to older drivers. The model that only considered data collected during LOS A conditions found the presence of roadway junctions (i.e., interchanges) to be significant. The authors created a similar dataset for two-lane highways that included 2,901 driver traces representing posted limits ranging from 25 mph to 60 mph. The authors commented that the interquartile ranges were found to be wider for two-lane highways, which is indicative of more diverse speed choices on those facilities compared to freeways, and that the difference in mean speeds between two consecutive limits seemed to be decreasing when reaching higher posted limits. The average speeds by posted speed limit for LOS A for the two-lane highways were: ï· 25 mph posted speed limit and LOS A: about 25 mph average speed. ï· 30 mph posted speed limit and LOS A: about 29 mph average speed. ï· 35 mph posted speed limit and LOS A: about 37 mph average speed. ï· 40 mph posted speed limit and LOS A: about 41 mph average speed. ï· 45 mph posted speed limit and LOS A: about 46 mph average speed. ï· 50 mph posted speed limit and LOS A: about 48 mph average speed. ï· 55 mph posted speed limit and LOS A: about 54 mph average speed. ï· 60 mph posted speed limit and LOS A: about 58 mph average speed. Savolainen et al. (179) found that mean speeds were generally near the posted speed limit under lower speed conditions (e.g., 25- and 35-mph limits) but tended to decrease below the posted limit at posted speed limits of 40 mph and above. Speeds were lower in the vicinity of access points (driveways and intersections) and in the presence of on-street parking. ONLINE REVIEW OF STATE GUIDANCE FOR SETTING SPEED LIMITS An online search identified 31 states with information on existing speed limit practices or guidelines for setting speed limits. The remaining 19 states either did not have design or traffic control device guidance online or the materials available did not specifically discuss the setting of speed limits. While many of those 19 states probably have guidance available, the 31 states identified should provide a representative review of the current practices regarding speed limits in the United States. The guidance materials included manuals, state statutes on speed limits, brochures, state versions of the MUTCD, and speed limit guidance documents. The documents reviewed included the following: ï· Alabama, Speed Management Manual (180). ï· Arizona, website (181). ï· California, Manual for Setting Speed Limits (182). ï· Colorado, Establishing Realistic Speed Limits (183). ï· Connecticut, Speed Limit Frequently Asked Questions and Highway Design Manual (184, 185).
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 60 ï· Florida, Speed Zoning for Highways, Roads, and Streets (18). ï· Indiana, Design Manual (186). ï· Iowa, Traffic Safety Manual, Chapter 5âSpeed Limits (187). ï· Illinois, Policy on Establishing and Posting Speed Limits on the State Highway System (188). ï· Kansas, Establishing Speed Limits: A Case of âMajority Ruleâ (189). ï· Kentucky, Traffic Operations Guidance Manual (190). ï· Louisiana, website (191). ï· Michigan, Establishing Realistic Speed Limits (192). ï· Missouri, Traffic Practices: A Guidebook for City and County Agencies (193). ï· Maine, How Are Speed Limits Set? (194). ï· Minnesota, Speed Limits in Minnesota (195). ï· Montana, Speed Limits (196). ï· Nebraska, How Are Speed Limits Established? (197). ï· New York, Traffic Control Program: Speed Limits (198). ï· North Carolina, Speed Limits (199). ï· North Dakota, Speed Limit Guidelines (200). ï· Ohio, Traffic Engineering Manual (201). ï· Oregon, Speed Zone Program: Interstate Speed Limits (202). ï· Pennsylvania, Official Traffic Control Devices (203). ï· Texas, Procedures for Establishing Speed Zones (204). ï· Utah, Establishment of Speed Limits on State Highways (205). ï· Vermont, Setting Speed LimitsâA Guide for Vermont Towns (206). ï· Virginia, Speed Limits and Guidelines for Interstate Speed Limits (207, 208). ï· Washington, Speed Limits (209). ï· Wisconsin, Traffic Guidelines Manual (210). Factors Considered, Especially 85th Percentile Speed For the 31 states where details regarding the setting of posted speed limits were found online, all considered the 85th percentile speed in the process, and most clearly stated that crash history should also be considered. Several states provided other factors to consider when setting a speed limit, with some states using specific terms (i.e., lane width, number of lanes, shoulder width), while other states used more general terms (i.e., driving environment, roadway geometry). Table 14 summarizes factors that influence speed limit setting decisions outlined in state guidance documents by more than three states. Factors mentioned by fewer than three states included (in alphabetical order) clear zone widths, commercial versus residential, consideration of adjacent speed limits, consistency, design speed used during project, driving environment, operational analysis (traffic volumes, percentage of trucks, congestion, etc.), existing traffic control devices, functional class, hidden driveways, land use characteristics, law enforcement, level of maintenance such as snow and ice removal, local versus regional traffic, narrow roadway pavement, number of lanes, presence of passing zones, right lane/entering traffic conflicts, roadside culture, roadway lighting, roadway width, roadside friction (number of driveways, parking, pedestrians, etc.), special features, surface type, traffic characteristics, traffic signal, transit vehicle activity, and type of area.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 61 Table 14. Factors typically considered by U.S. states in setting posted speed limits based on material found online for 31 states. Frequency Cited Factor All or most of states 85th percentile speed Crash history Over one- half of states Roadside development or land use Traffic (pedestrians, bicyclists) condition or volume Maximum or minimum speed allowed in state Sight distances About one- third of the states Parking practices Shoulder presence/condition/width Pavement or surface characteristics/condition Access (points such as intersections or driveways), access characteristics including number, type, and design of roadway and driveway intersections Less than one-third of the states, but greater than three states Functional class Grade Horizontal and/or vertical curves Lane width Pedestrian activity Roadway alignment Roadway characteristics Roadway geometry Roadway width Traffic control devices Transition zones Urban streets As illustrated in Table 14, the terminology of factors affecting the speed limit varied between states; for example, some states used the term âroadside development,â while others used âland use.â There were also other factors that were unique to some states that were not included. These factors are: ï· Design speed during project (Indiana). ï· Narrow roadway pavement (Texas). ï· Transit vehicle activity (Alabama). ï· Roadway lighting (Washington). ï· Width of clear zone (Wisconsin). ï· Local versus regional traffic (Wisconsin). ï· Presence of passing zones (Vermont). ï· Operational analysis (i.e., percent of trucks, congestion; Virginia, Wisconsin). ï· Consideration of adjacent speed limits (Virginia). ï· Level of maintenance such as snow and ice removal (Wisconsin). ï· Right lane/entering traffic conflicts for freeways (Washington). All states cited that crash history or crash rate influences the speed limit for a road segment. The following four states set a time requirement for the time frame that the crash history is to be analyzed: ï· Alabamaâ3 years minimum, 5 years preferred. ï· Kentuckyâ3 years.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 62 ï· Maineâ3 years. ï· Wisconsinâ3 to 5 years. Rounding of Speed Value The majority of the states that mentioned the use of the 85th percentile speed only gave the guideline that the speed limit should be within 5 mph of the calculated 85th percentile. Some states gave guidance for rounding the values obtained from the speed study. Below are a few examples: ï· CaliforniaâEstablish at or near the 85th, establish at the nearest 5-mph increment to the 85th, rounding as standard mathematics direct (182). ï· FloridaâThe set speed limit should not differ from the 85th percentile speed or upper limit of the 10-mph pace by more than 3 mph and it shall not be less than 8 mph (18). ï· MaineâSpeed limit is to be within 3 mph of 85th percentile, then rounded to the nearest 5 mph (194). ï· New YorkâEngineering judgment shall be used to adjust the value, but it shall not be lower than 3 mph than the upper limit of the 10-mph pace. Should not be lower than the 67th percentile (198). ï· North DakotaâWithin 5 mph of the 85th, shall not exceed statutory maximum speed, shall not be less than the 50th percentile (200). ï· TexasâThe final speed limit may be lowered as much as 5 mph from the 85th percentile. Posted speed limit is the nearest value ending in 5 or 0 (204). ï· UtahâAny reduction beyond rounding shall not be less than 5 mph from 85th percentile (205). COMPARISON BETWEEN EXISTING SPEED LIMIT SETTING APPROACHES As part of identifying potential variables to consider for the SLS-Procedure, the research team reviewed the variables being used in several approaches during Phase I of this research project. The approaches included USLIMITS2, New Zealand, Canada, and Portland, Oregonâs preliminary revised approach for collector streets. A preliminary dataset consisting of Austin, Texas, segments was used in the investigation. The same dataset was also used with the HCM free-flow speed equations. While the exercise produced suggested posted speeds or predicted speeds, the value of the effort laid in other areas. The exercise identified the variables being used in the calculations. It also provided experience in how to obtain the value for the variable such as how easy it was to obtain that value or whether an assumed value would be needed or sufficient. It also provided the opportunity to compare and assess the types of variables between the different procedures and to consider the sensitivity of a given variable to the overall answer. While not exhaustive in its analysis, the effort generated the following key observations: ï· Several variables used in the HCM free-flow speed prediction are considered in the suggested PSL procedures reviewed, including the following: access density, median type, number of through lanes, one-way/two-way operations, on-street parking, existing posted speed limit, section length, and traffic signal density. ï· A variable used in the HCM that was not used in the reviewed PSL procedures was proportion of length with curb on right-hand side. This absence may be more of a
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 63 reflection that the procedures examined focused on developed areas that would typically only have roads with curbs. ï· For the arterial streets available for the investigation, the HCM procedure resulted in calculated operating speeds greater than PSL when PSL is 30, 35, or 40 mph and operating speeds less than PSL when PSL is 45, 50, and 55 mph. Additional research or calibration may be needed because other research has found that operating speeds are higher than PSL for PSLs of 45 to 55 mph. ï· A variable that was used in the HCM but not in USLIMITS2, New Zealand, or Canada is the proportion of the segment with curb and gutter. ï· Several assumptions or engineering judgment had to be made to use the preliminary Portland procedure. The procedure determines a suggested PSL for vehicles, pedestrians, and bicyclists, and the lowest value is to be used as the recommended PSL. For the data used in the review and for the early version of the Portland procedure, the conditions for bicyclists dominated the results. This Portland procedure is to only be used on collector streets. ï· The New Zealand and Canada procedures base the recommended PSL on an assessment of the conditions at the site (e.g., road classification, function, physical characteristics, engineering factors) rather than on the measured operating speeds. Operating speed is used as a check to determine if additional measures such as engineering, enforcement, education, and publicity are needed. ï· While USLIMITS2 also considers a number of roadway characteristics, the procedure ultimately considers existing vehicle operating speeds to determine the recommended PSL. For example, high levels of signal or driveway density or pedestrian/bicycle or parking activity result in a recommendation of using a PSL closest to 50th percentile speed rather than 85th percentile speed. A comparison of the input variables considered in the various procedures is shown in Table 15 (for variables starting with A to C), Table 16 (for variables starting with L to O), Table 17 (for variables starting with P to S), and Table 18 (for variables starting with T).
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 64 Table 15. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables AâC). Input Variables Speed Prediction: HCM Suggested PSL: USLIMITS2 Developed Area Suggested PSL: Portland, 2016 version Suggested PSL: New Zealand Suggested PSL: Canada AADT Yes Access (or driveway) density Yes Yes: specific values Yes Alignment Yes: based on âpresence/ absences of adverse alignmentâ Yes: by âlimited, average, or open visibilityâ Yes: risk is higher when more curves or steep grades are present Area type/ development Yes: lower PSL when commercial or residential Yes: more points when residential Bike facility: lane width and separation (distance or barrier) Bike facility = shared road, by bike lane width, separated Bike facility = none, narrow road, wide road, separated Yes Classification Only for collectors (not for state highways or arterials) Yes: arterial, collector, or local Yes: arterial, collector, etc. Also: major or minor Crash statistics Considered in other parts of the USLIMITS2 Indicates when additional engineering/ enforcement is needed Curb Yes: proportion of length with curb on right- hand side Blank cell = variable not included in procedure
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 65 Table 16. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables LâO). Input Variables Speed Prediction: HCM Suggested PSL: USLIMITS2 Developed Area Suggested PSL: Portland, 2016 version Suggested PSL: New Zealand Suggested PSL: Canada Lane width Yes Yes: higher PSL when lane â¥ 3.5 m Yes: narrow, moderate, wide Lighting Yes: higher PSL when lighting is present Median type Yes: proportion of length with restrictive median Yes Yes: higher PSL when median â¥ 4.5 m or fully protected Yes: divided or undivided Number of through lanes Yes: affects adjustment factor for access density Yes, per user guide Yes One-way to two-way street Yes Yes, per user guide Yes On-street parking, presence or usage Yes: proportion of length with on-street parking Yes, per user guide Parking = none, obstruct traffic, do not obstruct, more than 6 ft from moving veh Frequency = none, rare, frequent (long or short duration) Yes Operating speed (85th and 50th percentile speed) Yes Yes: higher PSL when higher operating speed is present Blank cell = variable not included in procedure
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 66 Table 17. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variables PâS). Input Variables Speed Prediction: HCM Suggested PSL: USLIMITS2 Developed Area Suggested PSL: Portland, 2016 version Suggested PSL: New Zealand Suggested PSL: Canada Pavement surface Yes Pedestrian facility: sidewalk and separation (distance or barrier) Sidewalk = none or one side or two sides Separation = yes Pedestrian facility = none (i.e., on road), shoulder, sidewalk adjacent, sidewalk separated Yes Pedestrian/bicyclist activity Yes, per user guide Yes: for some cases NCHRP 562 (20 ped/hr) Yes: more or less than 200 ped/day and more or less than 200 bike/day Posted speed limit or statutory limit, existing (mph) Yes Yes, per user guide Presence of engineering to control speed Yes Roadside Yes: higher PSL with âroadside object setback or shieldingâ Yes: higher PSL when setback to fence line â¥ 6 m Yes Section length Yes: considered in signal and driveway density, and in curb and on-street parking proportions Yes, per user guide Yes Blank cell = variable not included in procedure
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 67 Table 18. Comparison of input variables used in speed prediction or suggested posted speed limit procedures for urban/suburban (or developed) areas (variable T). Input Variables Speed Prediction: HCM Suggested PSL: USLIMITS2 Developed Area Suggested PSL: Portland, 2016 version Suggested PSL: New Zealand Suggested PSL: Canada Traffic control devices Yes: considers the presence of several types of traffic control device, with more points assigned for presence of pedestrian crossing or stop control on the segment Yes: part of number of intersections with public roads, considers stop- controlled, signalized, roundabout, crosswalk, at- grade railroad, and side street stop-control intersections Traffic signal density or spacing Yes Yes: specific values Yes: part of number of intersections with public roads Blank cell = variable not included in procedure DISCUSSION OF SPEED LIMIT APPROACHES The operating speed (engineering) approach is the most common method used in the United States. It relies on the 85th percentile speed with adjustments used to account for existing roadway geometry or crash experience. Using techniques other than 85th percentile speed to select the posted speed limit is gaining in popularity for other counties. Several citiesâsuch as Portland, Oregon; Boston, Massachusetts; New York City, New York; Seattle, Washington; and othersâare also experimenting with alternative approaches for city streets. In the United States, there are two major trends with respect to speed limits. For some rural highways and freeways, there is a trend to increase posted speed limits. The other major trend is to lower speed limits, especially on city streets or in neighborhoods. Cities concerned with recent increases in pedestrian deaths or as part of their Vision Zero initiatives have explored options to improve safety for all road users. One of those options is a focused effort to lower speed on their streets. As part of these efforts, cities such as Boston, New York City, Portland, and Seattle have lowered their citywide default speed limits recently.