This article was first published on December 2, 2011. The information is still useful, so we're sharing it again.
Listen to any discussion of highway congestion and you will inevitably hear about Level of Service (LOS), which assigns a letter grade to the congestion level of road segments. Letter grades start with ‘A’ for free flow and run down to ‘F’ for “failing” (congested) roads. Simple enough.
Simple enough, except that it makes absolutely no sense whatsoever and is completely counter-intuitive.
The problem is that people hear about roads with grades of C, D, or E and think that means they are badly congested roads, because Cs, Ds and Es are bad grades in school. Traffic engineers often refer to streets with LOS D or E as “nearly failing,” which sounds bad to anyone who speaks English.
But that isn’t how it actually works. Any LOS above F is good. A road with an LOS of E is still moving very well.
Take a look at this year’s Metropolitan Washington Aerial Traffic Congestion Survey. Download the pdf and go to its 11th page, where LOS speeds are defined. This is what you will find:
LOS A, B, and C are all free flow conditions. LOS D equates to highways moving at 65 miles per hour. LOS E is 55 mph. A highway can receive a score of LOS F - failing - and still be moving at somewhere around 40 mph.
So for the record, a highway scoring LOS D is moving faster than the legal speed limit on most highways in our region. How completely ridiculous.
Don’t ever let anyone tell you Ds and Es are bad grades for highways. They aren’t.