Level of Service (LOS): Traffic and Parking Grades

Level of service (LOS) is a letter grade, from A to F, that describes how well a road, intersection or parking facility works from the user’s point of view.

The six grades from A to F describe the same lane at rising density. In parking work the grades are applied to entry and exit lanes, ramps and nearby intersections. Illustrative.

Parking uses the grades in two different ways. In traffic engineering, following the Highway Capacity Manual, LOS rates traffic flow by delay, speed and density, and it applies to the streets, entrances, exit lanes and ramps around a garage. In parking facility design, as set out in the National Parking Association’s Dimensions of Parking, LOS rates the comfort of the geometry inside the facility: stall and aisle widths, turning radii, ramp slopes and walking distances.

The traffic grades, A to F

LOS A: free flow, with almost no delay.
LOS B: reasonably free flow with slight restrictions.
LOS C: stable flow, with noticeable limits on speed and maneuvering.
LOS D: approaching unstable flow; a common design target for urban peak hours.
LOS E: at capacity, with unstable flow and significant delay.
LOS F: breakdown, with queues and stop-and-go conditions.

LOS in parking design

The designer chooses a target grade for the people the facility will serve, then sizes the layout to it. 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.

Unfamiliar, short-stay users such as retail customers, hospital visitors and airport travelers usually get a higher grade, A or B.
Familiar, long-stay users such as employees and residents can be served at C or D with tighter, more efficient geometry.
Each step up in comfort adds floor area per stall, so the grade is an economic decision as much as a design one. See parking geometry and parking module.

Where the two meet

A garage can have generous LOS A geometry inside and still fail at its entrance. For a parking facility, traffic LOS F usually shows up as queues backing onto the street or exit lines that take many minutes to clear after an event. Lane counts, transaction speed at the gates and peak spreading are the levers. See traffic study and peak demand facilities.

The Parkonomics view

A letter grade is a promise about how a place will feel. Choose it on purpose for the people who will use the facility, and check both kinds: the comfort inside and the flow at the door.