Level of service (LOS) is a measure used by traffic engineers to determine the effectiveness of elements of transportation infrastructure. LOS is most commonly used to analyze highways by categorizing traffic flow with corresponding safe driving conditions. The concept has also been applied to intersections, transit, potable water, sanitary sewer service, solid waste removal, drainage, and public open space and recreation facilities.

The following section pertains to only North American highway LOS standards and it uses the letters A through F, with A being the best and F being the worst.

Level-of-Service A describes free-flow operations. Traffic flows at or above the posted speed limit and all motorists have complete mobility between lanes. The average spacing between vehicles is about 550 ft(167m) or 27 car lengths. Motorist have a high level of physical and psychological comfort. The effects of incidents or point breakdowns are easily absorbed. An example of LOS A occurs late at night in urban areas, frequently in rural areas, and generally in car advertisements.