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Maryland Department of Transportation
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Are there any restrictions on how high or low speed limits may go?
Maryland law limits the extent to which speed limits may be raised or lowered. The most notable restrictions are those that prohibit any speed limit greater than 65 mph and any limit above 55 mph anywhere except on Interstate highways or other expressways.

What is a traffic engineering study?
A traffic engineering study is the observation and analysis of road and traffic characteristics to guide the application of traffic engineering principles. The study of speed limits includes the following:

  • Review of the road's environment, features, and condition and traffic characteristics.
  • Observation and measurement of vehicle speeds at one or more representative spots along the road in ideal weather and under free-flowing traffic conditions
  • Analysis of the vehicle speeds to determine average and 85th percentile speeds and other characteristics.
  • Review of the road's speed history.
  • Review of any unusual conditions not readily apparent to the driver.

What is the 85th percentile speed?
The 85th percentile speed is the speed at or below which 85 percent of the motorists drive on a given road when unaffected by slower traffic or poor weather. This speed indicates the speed that most motorists on that road consider safe and reasonable under ideal conditions. It is a good guideline for the appropriate speed limit for that road

Will crashes increase if the speed limit is raised?
Probably not. Research has shown that the posted speed limit has little effect on the speeds at which most motorists drive. Raising the speed limit does not significantly raise the speeds at which motorists drive, and lowering the limit generally does not appreciably decrease their speeds. However, the more motorists learn from their experiences that speed limits are set at speeds that they consider safe and reasonable the greater the chances that the motorists will heed them. Speed limits significantly lower than the 85th percentile speed are ignored by many drivers and difficult to enforced

In most instances, a speed limit based on the 85th percentile reflects the expectations of the largest proportion of drivers; is found by most to be

a safe and comfortable limit; facilitates speed enforcement; and offers the greatest chance of achieving some uniformity in speeds on a given road. When motorists drive at a relatively uniform speed, tailgating, lane changing, and overtaking are reduced. As a result, collisions are less likely to occur.

Those who drive much faster or slower than most of the drivers around them place themselves and others at considerable risk of a collision. When the posted limit is reasonable, enforcement can be targeted to the relatively small percentage that exceeds the speed limit.

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Revised: June 18, 2008