The vast majority of roadways in the United States are regulated by posted speed limits. Speed limits are typically posted in increments of five miles per hour (mph). There are four primary types of speed limits in the U.S.: statutory, prima facie, absolute, and advisory.
Statutory speed limits are set by state legislatures and signed into law. Statutory speed limits apply to all drivers regardless of roadway conditions or time of day. In most states, the default statutory speed limit is 55 mph on highways and 65 mph on freeways unless otherwise posted.
Prima facie speed limits are established by engineering studies that take into account factors such as curvature, grade, sight distance, and crash history. Prima faciespeed limits represent the maximum safe speed for a given stretch of roadway under ideal conditions (i.e., dry pavement, good visibility). Drivers must use caution when approaching work zones, inclement weather, or other adverse conditions as these can significantly reduce the safe operating speeds on a highway or freeway.
Absolute speed limits are established when it can be proven through engineering studies that driving faster than the posted limit would be inherently dangerous under any circumstances. Absolute speed limit signs are very rare in the United States; one notable exception is Montana’s “nighttime travel restriction” which prohibits driving faster than 45 mph between sunset and sunrise regardless of prevailing conditions (Montana Code Annotated 61-8-301).
Advisory speeds may be posted at locations where special hazards exist such as sharp curves or steep grades even though the design standards for those stretches of highway would not warrant an absolute or statutoryspeed limit reduction under normal circumstances. Advisory speeds serve as guidelines to help motorists navigate safely through these areas but are not enforceable like other types of Posted Speed Limits: Statutory - Set By State Legislature And Signed Into Law / Prima Facie - Established By Engineering Studies / Absolute - When It Can Be Proven That Driving Faster Would Be Inherently Dangerous / Advisory - When Special Hazards Exist But Don't Warrant Another Type Of Limit