Traffic calming design standards for new residential streets: A proactive approach

Institute of Transportation Engineers. ITE Journal, Mar 2003 by Womble, Joseph E, Bretherton, Martin Jr

Figure 3 shows a scatter diagram of the 35 studied curves. Most of the data points are left of the 30-mph marker, showing possible curve designs of less than 30 mph using different delta angles. There is a very strong correlation between delta angle and curve length (correlation coefficient equals 0.94), because the radius (or curve length) usually is not chosen independently once the delta angle is determined; it often is determined on the basis of design criteria. Due to the strong correlation between delta angle and curve length, the speed prediction model based on the regression analysis would have only one of the two as an independent variable.

Both analysis of variance and regression analysis were conducted to determine the relationship between operating speed and horizontal curve design on residential streets. No model could be found because all relationships were statistically insignificant (best R-squared equals 0.66). The study plotted all data points on a graph and drew a line at 30 mph, as shown in Figure 3. The reasonable grouping of data points was used. Based on this study, Table 2 shows curve values required to maintain operating speeds in the 25-30 mph range.

Breaks in Continuity. Conditions that require a motorist to come to a complete stop include a T intersection or a stop-- controlled intersection between a residential street and a collector or arterial road. These conditions do not include unwarranted multi-way stop control at an intersection between local subdivision streets. (Section 2B.05 of the Manual on Uniform Traffic Control Devices states that, "Stop signs should not be used for speed control." Experience has shown this to be a sound policy, which the Gwinnett County DOT supports).3

Traffic Calming Devices. While there are various traffic calming devices available, those now considered for use in Gwinnett County are limited to speed humps, traffic circles, median islands and roundabouts.4 Design details for these devices will be presented in the "Traffic Calming Guide for the Approved Design and Spacing of Traffic Calming Devices," currently under development. Design guidelines for roundabouts are contained in the Federal Highway Administration guidelines.5

TRAFFIC CALMING CRITERIA FOR

NEW RESIDENTIAL STREETS

With this research to serve as background, very simply stated criteria have been developed to govern low-speed design of residential streets in new developments. As such, subdivision streets should be designed to encourage and maintain 85th-percentile speeds in the 25-30 mph range. To achieve this objective, the maximum length of a roadway section between speed control points should be 500 ft. A speed control point is defined as any one of the following:

* Any design condition that requires a complete stop, such as the intersection of a local residential street with a collector or arterial road or a T intersection between local streets. (Unwarranted stop-sign control at an intersection between local streets does not qualify.)