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

The design elements considered in developing low-speed design criteria include tangent lengths and various types of speed control points, such as horizontal curves, breaks in continuity and different types of traffic calming devices.

Tangent Lengths

While numerous studies have been conducted to determine the effect of tangent lengths on operating speeds, additional studies were conducted to determine this relationship based on Gwinnett County's subdivision development standards (such as street widths, setbacks and parking conditions).

Accordingly, speed studies were completed on eight residential streets with 24 tangent sections. The studies were conducted over 24-hour periods with electronic tube counters. The accuracy of the counters was checked by radar. Tangent lengths ranged from 300 to 2,510 feet (ft.) and operating speeds (85th percentile) ranged from 25.5 to 41.2 mph. The studies were conducted at the midpoints of the tangents. Figure I shows a scatter diagram of the 24 studied road segments.

A regression analysis was conducted to determine the relationship between operating speeds and the length of tangent segments on residential streets. The model found the following relationship:

V = 16.6 0.03484 L - 0.0000138 L^

V = 85th-percentile speed (mph)

L = length of straight residential street (ft.)

The results of the application of this model, based on Gwinnett County's subdivision street standards, are presented in Table 1. Other results include the following findings:

* The model fits the data well with an R-squared value of 0.83. All residuals are within 1.5 standard error from the expected value.

* The model applies only to straight segments between 300 and 1,400 ft.

* The model found the 85th-percentile speed maximum value (38.6 mph) when the straight segment length is 1,260 ft. To be consistent with the theory that longer segment length generates higher speed, it was decided that the model would use a maximum value of 38.6 mph for segments longer than 1,260 ft.

Speed Control Points

Speed control points are defined as the design elements at the end of tangent sections that can be negotiated safely only at operating speeds of 25-30 mph or less. These include horizontal curves, breaks in continuity and traffic calming devices.

Horizontal Curves. As shown in Figure 2, the two most important curve characteristics influencing operating speed are delta angle and radius. (This assumes super-elevation rate e = o, which is the standard for Gwinnett County's residential streets.)

To determine the effect of horizontal curves on operating speed, a statistical analysis was performed on data collected on eight residential streets. The data included operating speed, delta angle and radius for 35 horizontal curves. The curve data were obtained from final subdivision development plats. Operating speeds in the study ranged from 21.5 to 37.4 mph and were measured at the point of curvature or point of tangency to determine the effect of the curve on speed. In addition, data were collected using automatic 24-hour traffic counters with rubber tubes. Vehicles needed to hit the tubes perpendicularly to obtain accurate readings. The delta angles ranged from 37 to 164 degrees and the curve radii ranged from 51 to 426 ft.