Speed zones and the signs posted to advise motorists of the speed allowed by law for a specific area exist to regulate traffic and prevent accidents. You can see the signs on highways, boulevards, roads and streets throughout the United States. The signs indicate the speed limit in miles per hour (mph). National, state and municipal governments determine the speed limits in a particular area.
The History of Speed Limit Signs
Speed zone signs evolved as cars began to travel increasingly faster. When automobiles became the preferred means of travel in the United States, speed regulation was the responsibility of the individual states.
However, the signs were often difficult to see at night, until the mid-1920s, when they began to feature “cat’s-eye” reflectors. As a car makes its way down a road in the dark, its headlights illuminate the sign, allowing the driver to read it. Speed zone signs have been made the same way ever since.
A few years later, the signs’ shapes and colors began to become standardized. In the 1970s, the U.S. Congress enacted a national maximum speed limit of 55 mph, in part, because of the oil shortages at the time. In 1987, Congress brought the speed limit up to 65 mph nationally but repealed that decision less than a decade later when, once again, it gave states the responsibility to enact their own laws regarding speed.
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Allowable Speed Limit Under Favorable Conditions
Speed limit signs inform drivers as to the allowable speed limit under favorable conditions, so they can arrive safely at their destination. Drivers should slow down if weather, poor visibility, traffic congestion, pedestrians or cyclists are a factor. The signs are rectangular or square, generally black and white, and they highlight the optimal speed limit in large numerical symbols. You may also see signs that say “speed zone ahead,” “school zone” or “construction zone,” which let you know that you’ll need to start slowing down.
Types of Speed Limit Signs
In the U.S., there are two types of speed limit signs. They are color-coded to make their meanings clear:
- A regulatory speed limit sign is usually black, white and rectangular or square, with black lettering against a white background. The number shown is the legal, allowable limit on a particular road. That means, when conditions allow, you can drive at the speed on the sign, but you cannot legally exceed it. As an example, if you see a black-and-white sign that states 65 mph is the legal limit, you can drive 65, but not 75.
You may pass through several speed zones as you drive a specific route, depending on the population of the area. For example, on a rural road, you may see a sign that tells you the maximum speed is 65 mph. However, if you’re within city limits, a lower speed limit will likely apply.
- An advisory or recommended speed limit sign typically features black lettering over a yellow background, and it's usually rectangular or square. It may also include arrows or curves to illustrate the road conditions that lie ahead as well as the recommended speed limit. You can see advisory signs in places like off-ramps or when sharp curves are ahead.
Advisory signs warn drivers to stay at the speed posted, as it’s safer to do so. As an example, if you see an advisory sign of 25 mph, that’s the maximum suggested speed limit considered safe for that particular stretch of road. You can go over the speed posted, but it may be dangerous to do so. If you get in an accident because you went over the posted speed limit, a prosecutor could find you legally responsible by saying you were driving too fast for the conditions.