Montana Department of Transportation

Montana Department of Transportation


Traffic & Safety Engineering Bureau

Speed Limits

Speed limits are posted to protect the public by informing drivers of the authorized, allowable speed of motor vehicles upon a roadway. Common speed limits are typically statutory as stated in Montana Code Annotated 61-8-303.

Engineering Considerations

Speed limits signage is posted only after traffic and safety engineering study has been conducted and where applicable, approved by the Transportation Commission. Traffic and Safety Engineering considers the length and width of roadway, roadway type and condition, location of access roads and intersections, existing traffic control, sight distance, crash history, and traffic speed studies.

Engineering judgment is an important tool. To ensure a consistent approach to speed limits statewide a traffic investigator must use knowledge of national accepted principles combined with learned experience in assigning safe speeds. Concerns regarding posted speed limits along a particular roadway or area should be addresses with you local city or county government. The Traffic and Safety Engineer should be contacted regarding a state or federally funded roadways of concern. If you are unsure as to which agency has jurisdiction over a speed limit, contact the Traffic and Safety Engineering MDT Headquarters Office or your local District Office.

Traffic Investigation

An important part of a traffic investigation is the speed study. Drivers take into consideration the conditions of the roadway environment when determining their comfortable driving speed. Thus, the speed which the majority of the drivers consider prudent is an important factor.

Roadway data is collected using automated traffic counters at select locations during idea driving conditions. A technical analysis is done to determine the 85th percentile. This 85th percentile represents the speed of which people drive during idea conditions. Experience has shown that a posted speed limit near this value is the maximum safe and reasonable speed most drivers will travel.

Common Questions

If we put up a reduced speed limit sign won't that slow traffic down?
No. Signs are one of many factors motorists take into account when determining an appropriate safe speed. Before and after studies in many Montana communities, and across the country, show that simply lowering the speed limit does not change driver behavior.
Will raising the speed limit cause motorists to speed up?
No. Raising the speed limit with the consent of local officials simply aligns the speed limit with actual traffic operation during favorable roadway conditions.
Don't local officials have any input on what speed limit(s) are set?
Yes. Your local officials are instrumental in having a study conducted and provide valuable insight about the public's view on the speed limit.