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The design speed of a road is the maximum speed that a vehicle can safely travel on that road under perfect conditions. It is created based on what class the road falls under, the conditions of the road itself, and the conditions of the surrounding land. Other factors are also taken into consideration, such as the maximum speed allowed by law. While the design speed is created for safety, it is important to remember that this speed is only safe under perfect conditions. Heavy traffic and bad weather affect how fast a driver can safely travel along a given road.
When choosing the design speed of a road, many factors must be take into account besides the condition of the road itself. If the road tends to have a high volume of traffic, the speed is adjusted to take that into account. The actual operating speed of the road is also considered. Actual operating speed is how fast traffic generally drives, regardless of what the legal speed limit is for the road.
When considering the design speed of a road, the classification of the road is taken into consideration. Roads fall under one of three classifications: local roads, collector roads, and arterial roads. Local roads are those traveled by the local traffic on a day-to-day basis and generally have a slower speed limit. Arterial roads have the fastest speed and are designed for drivers covering a long distance. Collector roads are designed to connect local roads with arterial roads and have a speed that falls somewhere in the middle.
Design speed does not cover all vehicles on the road. Cars, for example, can travel faster than tractor-trailers. Often signs state the maximum speed limit for cars and may list a slower speed for tractor-trailers. This is especially common in areas with steep hills or sharp curves.
Weather is another factor to keep in mind while driving. While the design speed covers clear road conditions with no restricted visibility, often rain or snow can make a road slick and obscure the driver's vision. When bad weather occurs, drivers should go slower than the posted speed limit and leave extra room between cars for braking.