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Speed Limits - Restrictions

Are there any restrictions on how high or low speed limits may go?

Maryland law restricts how much speed limits may be raised or lowered and sets broad standards. For example, 70 mph is the maximum speed limit that can be established on any interstate highway or expressway. And 55 mph is the maximum speed limit that can be established on all other highways.

What is a traffic engineering study?

A traffic engineering study is the observation and analysis of road and traffic characteristics to guide the further application of traffic engineering principles. The study of speed limits examines 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.
  • Analysis of vehicle speeds to determine average and 85th percentile speeds, as well as other characteristics.
  • Review of the road's speed history.
  • Review of unusual conditions not apparent to drivers.

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 is 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 shows 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 experience that set speed limits are safe and reasonable, the greater the chance that motorists will heed them. Speed limits significantly lower than the 85th percentile speed are ignored by drivers and difficult to enforce.

In most instances, a speed limit based on the 85th percentile reflects the expectations of most drivers; that is:

  • a safe and comfortable limit
  • facilitates speed enforcement
  • offers the greatest chance of achieving 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 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.

speedchart - chart of Deviation from average speed & Relative Involvement Rate.It increases as the deviation increases