Did You Know: How Speed Limits Are Determined

Posted by on May 3, 2016

Speed LimitSpeed limits are used to set the maximum (or minimum in some cases) speed at which vehicles may legally travel on particular stretches of road.  Speed limits in the United States vary depending on jurisdiction, with 75 to 85 mph common on interstate highways in the Western United States and 65 to 70 mph common in the Southeast and Eastern United States. States in the northeast have lower maximum speed limits, with Washington D.C. being the lowest in the nation at 55 mph. States may also set special speed limits for trucks and night travel along with minimum speed limits.

Contrary to popular belief, speed limits are NOT posted to increase a jurisdiction’s revenues through traffic fine collection. The primary purpose of a speed limit is to notify drivers of the maximum speed that is reasonable and prudent for a roadway segment. Setting appropriate speed limits can reduce the probability and severity of crashes.

The ALDOT Director and the Director of Public Safety have joint authority to alter a speed limit on State highways on the basis of an engineering study, with the approval of the Governor, up to the maximum allowed by the statutory speed limits. Alabama State Statutes 32-5A-170 and 32-5A-171 lay out statutory speed limits which automatically govern in the absence of a posted speed, and are also the maximum allowable speeds for each road type.

Speed limits are established based on an engineering study that takes several factors into account. Some factors that can influence speed limit include: roadway geometry (curves, shoulder condition, number of travel lanes, lane width, and sight distance), roadside land uses (schools, parks, etc.), crash history, traffic volumes, pedestrian/bicycle/transit activity, and most importantly, the measurement and analysis of vehicle speeds.

The nationally recognized method for establishing speed limits is based upon the 85th percentile speed. This is the speed that 85% of traffic travels at or below along a roadway segment. Posting speed limits near the 85th percentile speed generally improves driver compliance.

Speed limits usually change when there is a change in roadway characteristics For instance, if you are traveling into a city where there is going to be more intersections, higher traffic volumes, and more driveways, you would expect a lower speed limit.

The public can make a request for a speed limit change to their local officials. If the City or County wants to pursue a speed limit change, they then need to obtain a speed study. Once the study is completed, it must be submitted along with the request through an ALDOT approval process. If a speed limit change is approved, the City or County is then responsible for passing an ordinance to formally establish the new speed limit.

Sain Associates has performed speed studies across the State of Alabama and assisted ALDOT with preparation of the Alabama Speed Management Manual.

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