Transportation engineers evaluate highways to set speed limits based on entrances, road use, and geometry. Residential and commercial entrances, intersections, and pavement conditions are considered. The resulting index is compared to traffic to determine if a lower limit is needed. If so, a sample of existing speeds is taken.
Speed limits set below the state speed limit are set by transportation engineers after a thorough evaluation of the stretch of highway or highway in question. There are generally three steps to the evaluation that involve the entrances, the use of the road and its anatomy.
The first step in setting speed limits is to assess the density of existing or potentially existing slipways on the highway. This is done by counting the number of driveways, as well as intersecting roads on each side of the road in 0.2 mile (.1 mile) segments.
When engineers are evaluating a highway for the purpose of setting speed limits, they must differentiate between residential and commercial driveways. Commercial entrances have more weight than residential entrances, and commercial entrances that generate more traffic have more weight than those that generate less traffic. Intersections are also taken into account and weighted according to their importance in relation to the road where speed limits are set.
Road geometry is a very important factor in determining proper speed limits. The geometry of the road includes the width of the road, its shoulders, and the condition of the pavement. Sight distance restrictions as a result of horizontal and vertical curves, known to laymen as hills, are also taken into account.
Each category above is scored, added, and divided by the length of the road segment. The resulting index is compared to the amount of daily traffic on the highway, known as the Annual Average Daily Traffic (AADT), to determine whether or not a lower limit is warranted on that highway. If it is found that the highway does not require a reduced speed limit, then the case is closed and statewide speed limits prevail.
If it is determined that lower limits are needed, particularly for safety reasons, a sample of the existing speeds of vehicles moving freely on the highway is taken. This sample is obtained using the same type of radar or laser gun that police use to enforce pre-set speed limits.
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