Highway signs can be a warning sign that the road is not safe.
They also can be an indication that the driver has not been following the posted speed limit.
The National Highway Traffic Safety Administration says signs should be visible from a distance of at least 30 feet.
The agency says that signs that are more than 30 feet should be posted to warn of the possibility of a crash, and they should be clearly marked and legible.
They should also be at least 12 inches wide.
The signs should indicate the following: The speed limit is 30 miles per hour or less.