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Drive down virtually any public road in America, and it won't be too long before you see a sign at the roadside. Speed limit signs are among the easiest to spot, although the number you can expect to see on those signs varies a lot between different road types and locations. The fastest speed limit sign in America is an 85 mph sign in Texas, but the vast majority instruct drivers to stick to much lower speeds.

Part of the reason that speed limit signs are so easy to spot is that they look the same wherever you go. Their appearance is governed by the Department of Transportation, which publishes the Manual on Uniform Traffic Control Devices (MUTCD). The manual contains strict rules about the size, color, and shape of each sign type, including speed limit signs.

These rules apply to any road that's open to public travel and does not have full-time access restrictions. The ownership of the road doesn't make a difference to whether the rules apply or not, with publicly owned and privately owned roads needing to follow the same guidance.

Public access is the key differentiator between roads where the rules apply and those that are exempt. The MUTCD states that "roadways within private gated properties where access to the general public is restricted at all times" do not have to follow federal signage rules.

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In many other instances, the rules set out by regulators must be followed. The manual lists examples including roadways within shopping centers and office parks, schools, universities, and recreational parks, as places where the standard rules all apply. These roadways are all intended to be used by the public, and so they must have signs that any driver will be able to instantly recognize.

If you're looking to place a speed limit on a private road, there are a few limitations worth keeping in mind. Firstly, although it's not illegal to post a speed limit on a private road, you'll need to be the owner of the road in order to do so. Owners are then also responsible for the maintenance of the sign.

The other main limitation is that a speed limit sign on a private road isn't necessarily legally enforceable. Rules can differ between different states and counties, but to take just one example, in North Carolina, law enforcement can't ticket a driver for exceeding a posted limit on a road that isn't part of the city street network. However, they can still potentially ticket a driver if they see a car driving on that road at a speed that officers deem unsafe for the conditions.

While speed limit signs on a private road can act as a deterrent for speeding delivery drivers or other road users, they're not the only tool that can be used to lower road speeds. Deploying traffic calming measures like speed humps has also been shown to effectively slow drivers down, although they can cost a lot to install.