Montana authorities post speed limits to alert drivers to the maximum legal speed on any given road. These include daytime speed limits, which are in effect from 30 minutes before sunrise until 30 minutes after sunset, and nighttime speed limits for all other times.
Montana authorities do not set speed limits arbitrarily. In fact, a speed limit is not posted in the state until after a study of traffic and engineering factors has been completed. The engineering factors considered in setting a safe speed limit include the width and length of the roadway, the type of roadway, sight distances for drivers and the locations of intersections, access roads and traffic control devices like stoplights and signs.
In addition to an engineering study, authorities also conduct a traffic investigation prior to setting a speed limit. One of the most critical parts of the traffic investigation is the speed study. In a speed study, automated counters are used to collect traffic data during ideal conditions for driving. This data is used to determine how fast motorists are actually driving on the road. Investigators then determine the 85th percentile of the speeds actually driven by motorists. This 85th percentile is taken as a starting point in determining a safe speed. Traffic investigators understand from experience that 85 percent of drivers generally will not exceed a safe speed under ideal conditions.
When a driver exceeds a posted speed limit that driver is exceeding the speed that authorities have determined is the maximum safe speed for that stretch of road. Speeding is negligent, and if speeding causes a car accident the negligent driver may be held liable for damages in a civil lawsuit brought by injured victims.
Source: Montana Department of Transportation, “Speed Limits,” accessed April 11, 2015