Level of Service (LOS) grades are a set of standardized categories used in transportation planning and traffic engineering to qualitatively describe the operational conditions of a roadway, intersection, or facility like a parking garage. These grades, ranging from A to F, provide a measure of the quality and efficiency of traffic flow, considering factors like speed, travel time, traffic interruptions, and comfort and convenience. Each grade reflects a different level of service experienced by users:
- LOS A: Represents free flow. Drivers experience little to no delay and have the highest degree of mobility and comfort. Traffic density is low, and speed is only slightly reduced from the maximum possible.
- LOS B: Slightly more congested than LOS A, but still represents reasonably unimpeded flow. Some restrictions on maneuverability may be experienced, and speeds start to be somewhat reduced.
- LOS C: Balanced flow. Speeds are closer to the posted limits, but maneuverability within the traffic stream is more restricted. Stable flow is maintained, but road users experience more noticeable restrictions.
- LOS D: Approaching unstable flow. Speed and freedom to maneuver are more limited, and the user experience begins to decline significantly. This level is often considered acceptable in urban areas during peak hours.
- LOS E: Represents capacity. Flow becomes unstable, and there are significant delays. Vehicles move at low speeds, and sudden stops are frequent. Some agencies accept it at peak hours in dense urban areas, although LOS D is the more common design target.
- LOS F: Represents forced or breakdown flow. This level is characterized by traffic congestion and stop-and-go conditions, with little to no movement for extended periods. Comfort and convenience are extremely poor, and delays are frequent and lengthy. These grades help in the planning and design of roadways, intersections, and facilities, aiming to achieve a certain desired level of service based on the specific needs and context of the area.
LOS in parking design
Parking designers use the same letter grades with a different meaning. In parking facility design, as set out in the National Parking Association’s Dimensions of Parking, level of service rates the comfort of the geometry: stall width, aisle width, turning radii, ramp slope and walking distance. The designer chooses a target grade for the users the facility will serve, then sizes the layout to it. Traffic-flow LOS, from highway capacity analysis, still applies to the entry and exit lanes, ramps and the streets around the garage.
In geometric design, A is the most generous layout and D the tightest that is still acceptable; designs are not targeted at E or F. Higher grades suit high-turnover, unfamiliar users; lower grades suit familiar, long-stay users. See parking geometry.