Geometric Design Guidelines to Achieve Desired Operating Speed on Urban Streets

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1 Geometric Design Guidelines to Achieve Desired Operating Speed on Urban Streets Christopher M. Poea and John M. Mason, Jr.b INTRODUCTION Speed control is often cited as a critical issue on urban collector and residential streets. These streets are intended to provide access and distribute local traffic between neighborhoods and arterial street systems. Low operating speeds are desirable for the safe accommodation of pedestrians, bicyclists, and access along these streets. Observed operating speeds, however, often exceed the intended speed of these urban streets based on the original design speed or posted speed limit. There is considerable research on the relationship of geometric design and operating speed and accidents for rural high-speed roadways. (1,2,~ There is, however, less empirical data on low-speed streets to corroborate or dispute the applicability of these guidelines developed for the high-speed environment. There are several that present guidelines for the design of lowspeed urban streets. These guidelines present recommended design speeds for low-speed urban streets and minimum design values for geometric elements based on these design speeds. As the chosen geometric design values exceed the minimums for a given design speed, the geometric features have less influence on the resultant operating speeds along a street. This paper presents interim analysis and findings from a Federal Highway Administration research study on the Relationship Between Design Speed and Vehicle Operating Speed. Under this study, data were collected on low-speed urban streets with varying roadway, roadside, and driver/vehicle characteristics. Low-speed is defined as vehicle operating speeds below 64 kilometers per hour (km/h). An operating speed approach to determine design speed was developed to investigate geometric guidelines that would assist in attaining operating speeds consistent with the intent of the low-speed environment. a Research Assistant, Transportation Operations Program, The Pemsylvania Transportation Institute, The Pennsylvania State University, University Park, PA ITE Member. USE OF DESIGN SPEED The current geometric design procedures are presented in A Policv on Geometric Design of HiEhwavs and Streets ~ published by the American Association of State Highway and Transportation Officials (AASHTO). AASHTO defines the design speed as the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern. Design speed is a fundamental criterion upon which the design of roadways is based. A designer will select a design speed based on the land use, topography, and function of the roadway. The geometric elements of the roadway are then selected based on this design speed. Design speed is often used to ensure design consistency of a roadway, (2) By selecting geometric features based on the same design speed, it is intended that the alinement of the roadway will match the driver s expectancy. If a geometric feature deviates from the preselected design speed, there is potential to introduce design inconsistency. For example, selection of a small curve radius along a section of roadway can create an inconsistency in the high-speed design. Similarly, the selection of large radius curves or long tangent segments with wide cross sections can create an inconsistency to drivers in the low-speed environment. The designer has the discretion to select larger than minimum design values. The design value selection for geometric elements, however, implies a design speed for a roadway. Although it may be thought that above minimum values will give a greater margin of safety, larger design values for individual geometric elements actually changes the intended design speed of the facility. Little information is available on how departures from the minimum values directly change the actual operation of urban streets. b Director, Transportation Operations Program and Professor of Civil Engineering, The Pennsylvania Transportation Institute, The Pennsylvania State University, University Park, PA ITE Fellow. 70 Institute of Transportation Engineers 65th Annual Meeting

2 In the low-speed environment, there are other factors that may limit the usefulness of the design speed concept. Several studies have investigated the development of the design speed concept, existing shortcomings, and possible alternate approaches.(1,4,~,2 A significant limitation is that ordy a few geometric elements are controlled by the selection of a design speed value. The designer selects horizontal alinement features (curve radius) and vertical alinement features (length of vertical curve) and available stopping sight distance along the roadway based on a preselected design speed. However, there are several other design elements in the low-speed environment that must be selected without the assistance of design speed. Some of these elements are lane width, gradient, roadside clearances, and access density. Because not all design decisions are related to the design speed concept, the designer does not necessarily have a mechanism to evaluate the operational consequences of design decisions. OPERATING SPEED APPROACH A primary goal of this research is to develop an operating speed approach for low-speed urban street designs. Selection of geometric elements and their design values would be based on the expected vehicle operating speed. By developing predictive speed models with this type of approach, designers will be given a method of examining the operational impact of design decisions. A predictive speed model based on the roadside, roadway, and land use variables will provide a feedback loop in the design process. If the predicted operating speed does not agree with the intended speed of the street, geometric design values can be changed and reevaluated for impact on operating speed. The goal is to apply geometric design criteria to lowspeed urban streets in order to attain low-speed operation. By using more restrictive geometry, the driver perceives a low-speed environment and reduces operating speed. The study further investigates which geometric design elements influence vehicle operating speed and the influential range of design values. This requires collecting vehicle operating speed data on streets with varying geometric roadway and roadside variables. There are several benefits to be derived from this operating speed design approach. If creating a lowspeed environment through selection of geometric elements lowers vehicle operating speeds, there is potential to increase the safety. Achieving an approach that will decrease speed differentials between vehicles, between vehicles and pedestrians/bikes, and between vehicles and roadside objects will potentially reduce the number and severity of these types of accidents. EXISTING DESIGN GUIDELINES There are several sources that present the planning and design of local streets. These guidelines typically use a minimum design value approach for most of the geometric elements. For a given design speed, the minimum allowable value is presented. There is design information for horizontal alinement, vertical alinement, and cross section. AASHTO The AASHTO Green Book discusses the geometric design of low-speed streets. Design speed are suggested to be between 30 and 48 km/h for local roads and above 48 km/h for urban collectors. Some of the horizontal alinement assumptions that differ in the low-speed environment are side friction factor and superelevation. In urban areas, drivers tend to accept a higher threshold of lateral acceleration. The use of superelevation is not always available due to surface drainage, cross streets, and matching the grade of adjacent properties.(z) ~ The Institute of Transportation Engineers has published several sources for guidelines on urban and residential streets.(5, 10,11,12) The information in these guidelines, ranges from planning policy to design values for pavement widths, curve radii, grades, stopping sight distance, and sidewalk widths. ASCE The American Society of Civil Engineers, the National Association of Home Builders, and the Urban Land Institute published a guide to general principles and design considerations for residential streets.(~) The publication was developed to improve safety, efficiency, cost effectiveness, livability, and community attractiveness in residential areas. The design values given do not significantly differ from AASHTO. Shaw (~ presents a comparison of three of the documents. He found these different publications 1995 Compendium of Technical Papers 71

3 difficult to compare due to the variation in criteria presented. Shaw did note that there were differences in minimum horizontal curvature for local streets. Establishing minimum horizontal curvature is important to maintaining the integrity of low-speed urban streets. DATA COLLECTION Vehicle operating speed data were collected at 27 sites on urban collector streets in central Pennsylvania. Both older, traditional sites with restrictive geometry and more recent suburban sites were targeted. A statistical y significant sample of approximately 100 vehicles was obtained at each site. Only free-flow, unimpeded vehicles were used in this study. Six magnetic sensors were placed at each site to record vehicle operating speeds along the corridor. At each site, the roadway (radius, grade, lane width), roadside (access, lateral obstructions, sidewalks), and land use variables were recorded. Each study site contained a horizontal curve and a tangent section. Previous research has shown the importance horizontal curvature plays in vehicle operating speeds.(1,2) A control sensor was placed in a tangent roadway section where geometric features were judged not to influence operating speed. This speed represents the free-flow speed a driver chooses given the type of roadway and environment. In addition, sensors were placed in tangent sections 150 feet before and after the curve and at the point of curvature, midpoint, and point of tangency of the curve. The data collection plan allowed individual vehicles to be tracked through the study sites. Changes in vehicle operating speed could be examined in relationship to the changes in roadway and roadside variables, This paper concentrates on how the 85th percentile speeds were affected by the different geometric roadway configurations and which variables were significant in explaining vehicle operating speed. The critical design speed was determined at each sensor by using the measured geometric features in the field and back-calculating from the AASHTO design equations. The critical design speed represents the design speed implied by the geometric design decisions. The two equations that controlled the critical design speed were the stopping sight distance equation and minimum radius curve equations. The critical design speed is the minimum speed resulting from these two calculations. The critical design speed was plotted against the 85th percentile speed for each sensor as shown in Figure 1. For design speeds below 60 km/h, the majority of sites had 85th percentile speeds above the critical design speed. In this range, however, there was strong correlation between the 85th percentile speed and the critical design speed. As critical design speed increases, there is greater variability in the vehicle operating speed. This indicates that the decision by designers to use geometric elements associated with higher design speeds will result in higher speed and greater speed variability. so Critical Design Speed (km/h) Figure 1. 85th percentile speed vs. critical design speed. 150 ANALYSIS An aggregate analysis of the vehicles crossing each sensor was conducted to determine the 85th percentile speed, mean speed, and speed variance. This analysis provided information on how the roadway and roadside data influence the distribution of speeds at a particular site. A statistical analysis was also conducted to examine the 85th percentile operating speeds. A database was developed at each of the six sensors for the 27 sites. Regression analysis was run for two models estimating the 85th percentile operating speed (V~~). The first model used critical design speed (DS) as a predictor of this operating speed. The best model is shown in equation Instituteof Transportation Engineers 65th Annual Meeting

4 V~~= (DS) (DS)2 (1) R* = 0.68 Regression analysis was also performed to determinethe relationship between the 85th percentile speed and degree of curvature (DC). Several studies have investigated this relationship for high-speed facilities.(~) These studies show both linear and polynomial models with tangent speed between 95 and 100 km/h and a negative coefficient for degree of curve. For the lowspeed data in this study, the tangent speed is closer to 60 km/h, also with a negative slope. The degree of curvature model did not explain as much of the variability in speed at all the sensors, as shown in equation 2. V= = (DC) + 0,0009(DC)2 (2) R2 = 0.64 Another model was developed for a reduced database for only the semors on curvilinear sections of roadway. Regression analysis was performed to determine the relationship between the 85th percentile speed and degree of curvature. This model is shown in equation 3. As expected, the degree of curvature explains a greater percentage of the variability in 85th percentile operating speed. This relationship is shown in Figure 2. Vg~ = (DC) (DC)2 (3) R2 = I L o A 50 Inil... Degree of Curve I I Figure 2. 85th percentile speed vs. degree of curvature, A final model was run to examine all the variables collected. Other variables included were lane width (LW), hazard rating (HZ, a measure of the number and severity of lateral obstructions within 1.5 meters of the roadway), number of intersections (IN), number of driveways (DR), and grade (G, which is the absolute value of the roadway gradient). The best model is shown in equation 4. V,, = (DC) (G) (HR) (IN) (DR) + O.15(LW) (4) R2 = 0.67 Degree of curvature, hazard rating, and grade all were highly significant (significant at a 95 percent confidence level). The number of driveways, number of intersections, and lane width was less significant, but did show intuitively correct coefficients. This equation was developed to estimate the 85th percentile speed at the sites contained within this study. Each of the variables decreases the operating speed as the variables increase, except for lane width which increases speed with larger lane widths. FINDINGS The data were generally clustered in two ranges. Sites with more restrictive geometry had lower mean operating speeds than the sites with less restrictive geometry, The data were summarized to qualify the geometric features that resulted in these lower operating speeds. Based on this database and the analysis, design values were developed for two ranges of low-speeds. These design values are ranges for the data collected in this study. Attempts have not yet been made to develop predictive equations to apply at other sites. The critical design variables influencing operating speed are summarized in Table 1. The findings from this study give guidance to designers on the magnitude of design values to use in achieving desired operating speeds. As can be seen in Table 1, there were no sites with curve radii above 60 meters that resulted in mean operating speeds below 40 km/h. If planners and designers desire operating speed below 40 km/h, then this data indicate that sections with horizontal curvature must have a maximum radius of 60 meters. Similarly, the sites with higher mean speeds were associated with longer sight distances, flatter grades, and wider and more open cross sections. The data suggest that for design speed ranges there 1995 Compendium of Technical Papers 73

5 should be maximum curve radii, maximum widths for cross sections, and guidance on the effects of sight distance provided. Table 1. Design Values For Geometric Elements mvehicle Operating Speed Range Curve Radii (m) Sight Distance (m) Absolute Value Grade Lane Width (m) Parking none none none none Availability Lateral Clearance o Notes: Lateral clearance defined as follows: O = no obstructions, 3 = some flexible and fixed objects witbin 1.5 meters of roadway, 4 = continuous objects witbitr 1.5 meters of roadway. CONCLUSIONS Geometric design decisions have a significant influence on the operating speed of vehicles on low-speed urban streets. Careful selection of design elements will assist in maintaining the intended speed of urban streets. Designers need to be more aware of the operational effects of their decisions. The selection of above minimum design values for geometric elements will create an environment that appears suitable for higher speeds, and thus, may result in higher operating speeds. There is a need to establish maximum values in each design speed range for variables such as curve radius to maintain low-speed operation. Acknowledgements Funding for this research was provided by a grant from the Federal Highway Administration titled Relationship Between Design Speed and Vehicle Operating Speed. REFERENCES 1. FHWA, Horizontal Alinement Design Consistency for Rural Two-Lane Highways, Publication No. FHWA-RD , Federal Highway Administration, Washington, DC, January Larrtm, Ruediger, and Elias M. Choueiri, Recommendations for Horizontal Design Consistency Based on Investigations in the State of New York, TRR 1122, Transportation Research Board, Washington, DC, Islam, M. N., and P.N. Seneviratne, Evaluation of Design Consistency of Two-Lane Rural Highways, ITE Journal, Institute of Transportation Engineers, Washington, DC, February McLean, J, R,, An Altermtive to the Design Speed Concept for Low-Speed Alinexnent Design, TRR 702, Transportation Research Board, Washington, DC, ITE Technical Council Committee 5-5, Guidelines for Urban Major Street Design, Institute of Transportation Engineers, Washington, DC, American Society of Civil Engineers, National Association of Home Builders and Urban Land Institute, Residential Streets, Second Edition, ASCE, Washington, DC American Association of State Highway and Transportation Officials, A PoliV on Geometfic Design of Highways and Streets, Washington, DC, McLean, J. R., Review of the Design Speed Concept, ARR 8(1), Australian Research Board, Leisch, J., and J. Leisch, New Concepts in Design- Speed Application, TRR 631, Transportation Research Board, Washington, DC, ITE Technical Council Committee 5A-25A, Guidelines for Residential Subdivision Street Design, Proposed Revisions to a Recommended Practice, Institute of Transportation Engineers, Washington, DC, 1990, 11. Homburger, Wofgang s., et al. Residential Street Design and Traffic Control, Prentice-Hall, Englewood Cliffs, New Jersey, ITE Technical Council Committee 5P-8, Traffic Engineering for Neo-Traditional Neighborhood Design, An Informational Report Institute of Transportation Engineers, Washington, DC, , Shaw, Gordon R., Impact of Residential Street Standards on Neo-Traditional Neighborhood Concepts, ITE Journal, Institute of Transportation Engineers, Washington, DC, July Institute of Transportation Engineers 65th Annual Meeting