Speed Limits
Speed limits frame expectations for drivers and other roadway users. Properly set speed limits provide a safe, consistent, and reasonable speed to protect drivers, pedestrians, and bicyclists along the roadway.
At the same time, speed limits can be a source of frustration and confusion. For example, not all drivers like to travel at the same speed, and some people may not understand why the speed limit changes on a particular road.
In addition, community residents often have concerns that traffic is moving too fast through their neighborhoods.
Understanding the engineering principles and processes used to set speed limits, and learning the terminology used to describe them, are the first steps in reducing drivers’ frustration or confusion and encouraging compliance.
二、Types of Speed Limits
1. Statutory Speed Limits
Statutory speed limits are established by state legislatures for specific types of roads (e.g. interstates, rural highways, urban streets) and can vary from state to state.
They are enforceable by law and apply even if a speed limit sign is not posted.
Examples include：
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25 mph in residential or school districts
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55 mph on rural highways
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70 mph on rural interstate highways
2. Posted Speed Limits
Posted speed limits (sometimes called regulatory speed limits) are those posted along the road and are enforceable by law.
A posted speed limit may be the same as the statutory speed limit set by the state legislature, or it may be adjusted by a city, county, or state transportation agency.
Some cities and counties establish blanket speed limits that apply to all roads within their jurisdiction. These limits are generally posted at city or county boundaries.
The posted speed limit may differ from the statutory speed limit. In those cases, the posted speed limit is determined using an engineering speed study and takes priority over the statutory speed limit.
三、“Special Conditions” Speed Limits
3. School Zone Speed Limits
School zone speed limits are used in specific locations during the hours when children are going to and from school.
Most states use a school zone speed limit of 15–25 mph in urban and suburban areas.
4. Work Zone Speed Limits
Work zone speed limits are set as part of a work zone’s traffic control plan to help ensure safe and efficient movement of traffic through the work zone.
Factors that influence work zone speed limits include：
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The posted speed limit when the work zone is not present
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The location of the work zone and workers in relation to traffic
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The type of traffic control (e.g. cones, barrels, concrete barriers)
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The complexity of the work zone (e.g. lane shifts, narrowed lanes)
5. Variable Speed Limits
Variable speed limits are displayed on changeable message signs (CMS) at locations where roadway conditions regularly require speeds to be reduced more than 10 mph below the posted speed limit.
These conditions typically include：
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Weather
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Congestion
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Traffic incidents
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Work zones
6. Advisory Speeds
Advisory speeds are non-regulatory speeds posted for a short or isolated section of roadway (such as a sharp curve or exit ramp).
They are used to inform drivers of a safe driving speed and are set using an engineering speed study in accordance with guidance in the Manual on Uniform Traffic Control Devices (MUTCD).