Speed limits…if these haven’t driven you a tad crazy, you haven’t been in public service long enough. For very understandable reasons, our residents appear at public meetings on a regular basis to request and even demand a change in speed limit, usually because they expect it will address safety issues on their streets. Certainly, properly developed speed limits combined with proper signage that is maintained and enforced can increase safety, but arbitrarily developed speed limits can actually make matters worse.
There is a process for establishing speed limits and by following it and then properly signing the area, we increase the likelihood that police can enforce those limits.
The Delaware Motor Vehicle Law (Title 17, Chapter 147) says that any traffic control device that is not in compliance with the Delaware Manual on Uniform Traffic Control Devices (MUTCD) shall be unenforceable. Check with your legal counsel and your chief of police, but that means your sped limits must be established in accordance with the Delaware MUTCD.
The Delaware MUTCD[1] provides standards and guidance for establishing speed limits for public streets. The Delaware Motor Vehicle Law (Title 21, Chapter 41)[2] establishes (where no special hazard exists) a maximum speed limit of 25 miles per hour (mph) for business and residential districts. It also sets a speed limit of, “20 miles per hour at all school zones where 20 mph regulatory signs are posted and state the time periods or conditions during which the speed limit is in effect; such conditions may include when children are present or while one or more warning lights flash.”
Title 21, in §4170, does permit in some instances for local authorities to establish within their jurisdictions higher speed limits than those prescribed in §4169. Local authorities may prescribe a lower speed limit through the type of engineering study outlined in the Delaware MUTCD at section 2B.13.[3]
An engineering study for speed limits is primarily based upon an examination of existing, free flowing speeds to determine the 85th percentile speed.[4] However, a number of other factors are appropriate considerations to calibrate the 85th percentile findings, including:
- Road characteristics, shoulder conditions, grade, horizontal and vertical alignment, and sight distance;
- Pace speed;[5]
- Roadside development and environment;
- Parking practices and pedestrian activities; and,
- Reported crash experience for at least a 12-month period.
The first piece of this, determining the 85th percentile speed, strikes some as a sort of “mob rule” where the riskiest drivers are able to elevate the absolute speed permitted for a stretch of road. Properly done, this is not the case. The qualitative factors (A through E above) are used to calibrate for those roadway characteristics that may not be reflected in the speed data. In addition, utilizing the 85% speed relies on the experience that most drivers attempt to be prudent in the operation of their vehicles.
Speed data can be obtained in a number of ways. Pneumatic tube counters can be stretched across the travel lanes and collect not only the speed of each vehicle, but the timestamp, the type of vehicle, and the number of axles. Autonomous radar devices collect similar data, with or without feedback features for the driver (“Your Speed Is…”). Both of these types of instruments can generally collect a week of data or even much more, resulting in a robust data set of every vehicle, 24 hours per day, categorized into exactly when each data point occurred. This is especially helpful in directing targeted speed enforcement by the police.
Sometimes these devices are not available and it is acceptable to collect spot speed data. Where practical, a minimum of 30 vehicles and 30 minutes should be observed, in keeping with guidelines from the Institute of Transportation Engineers. Observing at least 30 vehicles at a given location will generally yield a 99.7% confidence level in the validity of the data within ±3 mph. Increasing the sample size to 60 vehicles increases this same validity to nearly ±2 mph.[6]
Regardless of the data collection method, periods of congestion should be avoided, since speeds will be artificially low. Instead, free flow periods of traffic will yield the most representative data.
Care must be taken, especially with this method, to use devices and personnel that do not alter the natural behavior of the drivers. For spot speed data, the Chief of Police, in full dress uniform, standing next to his/her marked police cruiser alongside the road, with a radar gun in hand is unlikely to yield the true nature of driver behavior at other times. However, there are a number of devices available for as little as $200-250, accurate to +/- 1mph, and about the shape and size of a cell phone that can be used surreptitiously on the roadside with great effect.[7]
For some local streets, the volume is sometimes so low that thirty vehicles will not be observed even after 60-90 minutes of data gathering. In those cases, pneumatic tube or radar devices used over a longer period would create greater confidence in the analyses.
The data are then compiled in any number of ways (a spot speed data analysis is shown below) to yield what is, effectively, a starting point. In this example, 115 speed observations were made in sixty minutes and the 85th percentile speed was calculated to be 27 mph. The MUTCD instructs us that speed limits shall be displayed in multiples of 5 mph,[8] so we begin with likely choices of 25 mph or 30 mph.
Next, we turn to the qualitative factors (A-E above) to further inform what the drivers have concluded in their use of the road. The pace speed is 18-27 mph, which probably points us down. If the roadway is narrow, if there is parallel parking, if homes are located close to the street, if many pedestrians and/or cyclists are observed, or if there are sight distance challenges, we will also want to move down rather than up. If there are crash reports from the last year or so, they should be examined to determine the nature of the crashes and whether speed was a factor.
Because we lack other details about this fictitious example, we can’t be sure, but the qualitative factors suggest that we should move down to 25 mph rather than up to 30 mph. Other best practices to consider include consistency along a route (i.e., not unnecessarily raising and lowering speeds if they can remain at a constant within the quantitative and qualitative factors) and consistency between similar streets within an area.
Finally, the engineering study described here and required by the MUTCD must be performed by a Professional Engineer licensed in the State of Delaware in accordance with Title 24, Chapter 28 of the Delaware Code.[9] The Delaware T2/LTAP Center’s Municipal Engineering Circuit Rider is intended to provide technical assistance and training to local agencies and so if you have MUTCD questions, need a speed study, or other transportation issues, contact Matt Carter at matheu@udel.edu or (302) 831-7236.
[1] https://deldot.gov/Publications/manuals/de_mutcd/index.shtml
[2] https://delcode.delaware.gov/title21/c041/sc08/index.shtml, §4169
[3] The MUTCD at Section 2B.13 states that, “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” (emphasis added).
[4] The 85th percentile speed is greater than or equal to the speed at which 85% of existing motorists feel comfortable and safe, as evidenced by their observed speeds. By corollary, 15% of motorists exceed this speed. This technique has gained acceptance in the transportation engineering field, has been studied exhaustively, and is the primary basis for speed studies in the federal and Delaware MUTCD.
[5] From the MUTCD – pace speed is “the highest speed within a specific range of speeds that represents more vehicles [counted] than in any other like range of speed. The range of speeds typically used is … 10 mph.”
[6] H. Douglas Robertson, ed., Manual of Transportation Engineering Studies, Institute of Transportation Engineers (ITE), Washington, DC, 2000.
[7] By standing (plain clothes) on the sidewalk with a colleague, a safety vest is not necessary, and you can easily create the appearance of one friend talking while another rudely check’s his/her phone, a scene which unfortunately will not seem at all out of place these days.
[8] Delaware MUTCD, Section 2B.13, ¶02
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