What happens if you are on a highway that does not have a speed limit posted? Is it different for paved/gravel roads? Under Michigan’s Vehicle Code MCL 257.628, the statewide maximum speed limit on all unposted highways is 55mph. This includes gravel roads. However, you must be careful that while you are on gravel roads that you must adjust to the conditions of that road. This means that just because you are on an unposted highway, if the road is in a condition that is not ideal, whether thru traffic, weather, or conditions, you must adjust to a speed that is reasonable in that circumstance.
This makes for a very broad law in determining what is reasonable under the circumstances. This is one of those very interpretation laws because to one person, what is considered safe driving on a gravel road may be different to another. Also depending what car you are in, day and time, etc. It makes it very difficult to judge because you have to kind of place yourself in police officers shoes and try to decide what they would consider to be unreasonable.
Remember, this law is only for unposted HIGHWAYS. According to Mich. Comp. Laws 221.20, a highway is any roads that are 4 rods (lanes) in width, and have two lanes on either side of a line that is run down the center of the road.