Traffic Jam Ahead: Level of Service Explained

This post by RCA President Colin Mills was originally published in Reston Patch.

Last week’s post about office space per worker and its implications for Reston’s planning received a very favorable response.  I was happy to take a complex subject and make it understandable to the average person.  One of RCA’s mission is to keep citizens informed on issues that matter to our future.

One response I got contained a request for a layman’s explanation of traffic Level of Service.  This bit of jargon gets thrown around a lot in planning discussions, and it seems like an important measure of traffic.  But what exactly is it, and what does it mean in the context of Reston’s planning?   

The concept of Level of Service is simple enough to explain.  The different Levels of Service (LOS) are each assigned to a letter, from A to F.  These operate like old-fashioned school grades: A is the best and F is the worst.  (Unlike with school grades, there is a Level of Service E.) 

These “grades” are applied to intersections.  They measure the average vehicle delay, or the amount of time a car will have to wait before making it through an intersection.  So for intersections controlled by traffic lights, Level of Service A means that the average vehicle waits less than 10 seconds to get through the intersection.  By comparison, LOS C means that the average wait is between 20 and 35 seconds, and LOS F means the average delay is more than 80 seconds.

How do we know what the “average” delay is?  For existing conditions, this is based on traffic counts.  By figuring out how many cars that go through a given intersection over a period of time, it’s possible to calculate the typical delay.  For future conditions, transportation engineers use sophisticated modeling software that starts with current conditions and then projects future traffic counts based on the projected future growth and development. 

Easy enough so far, right?  But there are some complicating factors.  First, remember that Levels of Service are applied to intersections, not roads.  So there is no Level of Service that applies to, say, Reston Parkway as a whole: rather, each of its intersections has its own LOS. 

Also, we’re interested in service during the peak periods, when the most vehicles are on the road.  It’s nice that the intersections are free and clear at 3 AM, but for planning purposes, we care most about the times when people are getting to or from work.  We often call this “rush hour,” but as most commuters know, nowadays it lasts longer than an hour.  The peak period lasts about 3 to 4 hours in the morning and again in the evening; roughly 7 to 10 AM and 4 to 7 PM.

So if Levels of Services are like grades, we should be trying for an A, right?  Not necessarily.  As populations grow and more cars join the roads, LOS A becomes unrealistic.  You simply can’t build in enough road capacity to keep traffic flowing that quickly in urbanized areas.  Most urban areas set the LOS standard at D or E.  The current LOS standard in Fairfax County is a D (average delay of 35 to 55 seconds).  The County is considering moving to LOS E (55 to 80 seconds) for areas around Metro stations.  (Why the drop?  Because they’re trying to encourage walking and biking to the stations.  Pedestrians and bicyclists move more slowly than cars, so giving them time to cross the street means additional intersection delays for vehicles.)

So how will Reston do on the Level of Service scale?  The County’s Department of Transportation did modeling to gauge the impact of the Task Force’s current development plan (Scenario G) on Reston’s traffic.  The results were not good.  The worst intersections, as you would expect, are on either side of the Toll Road.  These are considered Reston’s “gateway” intersections: where Sunset Hills and Sunrise Valley cross Wiehle, Reston Parkway, and the Fairfax County Parkway.

Currently, those intersections are at LOS D or E during the peak periods.  If Scenario G is built out as planned, every one of those intersections except Wiehle/Sunrise Valley would earn an F.  The average delay at those 6 “gateway” intersections would be over 2 minutes each.  The worst of the worst, Wiehle/Sunset Hills, is forecast to have an average delay of over 4 minutes during the evening peak.  An F doesn’t adequately describe that kind of backup; that’s an F-, or even F--.

It gets worse: Those projected delays assume that the County invests heavily in road construction and improvement, bus transit, and pedestrian and bike access.  The forecast roadway improvements alone would cost more than $500 million.  You may have noticed that governments aren’t exactly flush with cash; where’s the money going to come from?

Also, those calculations don’t take into account “spill back.”  That’s what happens when an intersection is so congested that the traffic backs up through previous intersections.  If you’ve tried to go south on Reston Parkway during the evening rush, you’ve experienced this first-hand.  The intersection at Sunrise Valley backs up, which creates a jam for cars trying to get over the Toll Road, which worsens the backup at Sunset Hills, which creates gridlock going up past the Town Center.  And that’s today; imagine how much worse it could get!

This kind of traffic nightmare is what RCA is worried about, which is why we keep banging the drum about the traffic impacts of the Task Force scenarios.  Should traffic be the only factor driving Reston’s future development?  Of course not.  But given the amount of time most of us spend in the car, it’s an important quality of life factor.

We accept a Level of Service goal of E around the station areas; encouraging walking and biking to the stations is important, and aiming higher is probably not realistic.  But E is not F, and it’s certainly not F--.  If you’re going to aim for a standard of E, make it count.  That’s why we argue that the revised Comprehensive Plan should specify that development is only allowed if it allows the 6 gateway intersections I mentioned earlier to maintain LOS E during peak periods.

I hope this post has shed some light on Level of Service for you non-traffic experts out there.  If you have any questions, feel free to leave a comment.  And rest assured that RCA will keep fighting to ensure that Reston’s future growth doesn’t bring our roads to a halt.