What specific rating, using letters like A to F, tells us how well traffic is moving on a road?

The specific rating, using letters like A to F, that tells us how well traffic is moving on a road is called Level of Service (LOS). This system provides a qualitative measure of operational conditions within a traffic stream, based on factors such as speed, travel time, freedom to maneuver, traffic interruptions, and driver comfort and convenience. The Level of Service is a standard concept in traffic engineering, primarily defined by the Highway Capacity Manual. Each letter grade signifies a different quality of traffic flow:

Level of Service A represents excellent traffic conditions, characterized by free-flow operation with very low traffic volumes, high operating speeds, and virtually no delays, allowing drivers complete freedom to choose their desired speed and lane.

Level of Service B indicates very good traffic conditions, where flow remains stable with slightly restricted speeds and minimal delays. Drivers still experience good comfort and reasonable freedom to maneuver, though it is slightly less than LOS A.

Level of Service C describes good traffic conditions, where flow is stable but speeds and maneuverability are restricted by higher traffic volumes. Drivers experience acceptable delays, and some restrictions in lane changes become noticeable.

Level of Service D signifies fair traffic conditions, with traffic flow approaching unstable conditions. Operating speeds are tolerable, but considerable delays are experienced. Drivers have limited freedom to maneuver due to the density of traffic.

Level of Service E represents poor traffic conditions, indicating unstable flow where traffic volumes are near or at the roadway's capacity. Speeds are very low, significant delays occur, and drivers have very little freedom to maneuver, with limited ability to change lanes.

Level of Service F denotes failed traffic conditions, also known as forced or breakdown flow. This occurs when traffic demand exceeds the road's capacity, resulting in extensive queues, very low stop-and-go speeds, and unacceptable delays. Traffic movement is highly constrained, often leading to gridlock.