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@Froggie is pretty much right. Freeway LOS is based primarily on vehicle density, with traffic over 45 passenger cars per mile per lane rated as LOS F. LOS C, for comparison, is up to 26 pc/mi/ln. As you can imagine, density does correlate with speed, but even at LOS E, vehicles are still moving at 50mph or more.

LOS for signalized intersections are based on completely different criteria. The letter grade is assigned based on average signal delay experienced per vehicle. Basically, at signalized intersections, if the average delay per vehicle is greater than 80 seconds it is rated at LOS F. LOS C is anywhere from 20 to 35 seconds of delay per vehicle.

One important thing I would point out is that the same freeway segment (or signalized intersection) can be rated at LOS F at, say 4:30 p.m. on a weekday but be at LOS C at 8:30 p.m. and LOS A at 3 a.m. and LOS B at 2p.m. on a Sunday. Unfortunately, most traffic studies are undertaken by first determining the peak (worst) morning hour and evening hour and ONLY evaluating those two hours. Sometimes, a midday peak hour or a Saturday peak hour is included depending on the situation (Saturday for a heavy retail area might be worse than any weekday peaks). Basically, nearly every traffic study done is basing its conclusions on studying peak hours, even if everything operates at acceptable LOS the other 22 hours of the day.

And yeah, this is only for vehicles. There are some LOS standards for pedestrians, but they aren't considered well-defined or particularly useful in the traffic engineering world form what I've seen. I do hope to see seriously upgraded methods for this soon as they really need to be quantified, in my opinion.

by Thom on Dec 2, 2011 8:13 pm • link • report