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Speed limits set below the statewide speed limit are established by transportation engineers after a thorough evaluation of the stretch of road or highway in question. There are generally three steps to the evaluation which involve the driveways, use of the road and its anatomy.
The first step in establishing speed limits is to evaluate the density of existing or potentially existing driveways on the road. This is done by counting the number of driveways as well as intersecting roadways on each side of the road in .1 mile (0.2 km) segments.
When engineers are evaluating a roadway for the purpose of establishing limits for speed, they must differentiate between residential and commercial driveways. Commercial driveways are given more weight than residential driveways and commercial driveways that generate more traffic are given more weight than those that generate less traffic. Intersections are also taken into consideration and weighted on their importance relative to the roadway where speed limits are being established.
The road geometry is a very important factor in determining proper speed limits. The geometry of the road includes the width of the roadway, its shoulders and the condition of the pavement. Sight distance restrictions as a result of both horizontal and vertical curves, known to layman as hills, are also taken into consideration.
Each category above is then scored, added up and divided by the length of the roadway segment. The resulting index is then compared to the amount of daily traffic on the road, which is known as Average Annual Daily Traffic (AADT), to determine whether or not a lower limit is justified on that roadway. If it is found that the roadway does not require a reduced speed limit, than the case is closed and the statewide speed limits prevail.
If it is determined that lower limits are needed, particularly for safety reasons, then a sample is taken of the existing speeds of free flowing vehicles traveling the road. This sample is obtained by using the same type of radar or laser gun that police use to enforce speed limits that are already determined.