Mr. Traffic Engineer – What is Level of Service?

Traffic engineers grade the operation of an intersection as A, B, C, D, E, or F based on the amount of time each vehicle has to wait to go through the intersection during a particular hour.  We use grades to give non-engineers a feel for how the intersection operates.  We don’t want to get hung up on fractions of seconds.  Every jurisdiction I know of considers Level of Service (aka LOS) A, B, and C to be acceptable.  Most jurisdictions in Minnesota considers LOS D to be acceptable, but LOS E and F are both unacceptable.  Some places, like Los Angeles, consider LOS F ok depending on how bad it is (the seconds of delay calculated).  You need to check with the jurisdictions who control the intersection.  I have been burned a few times by "unique" standards different jurisdictions have in their policies or ordinances.  Checking the local standards is now one of the first things I do on a new project.

For you technical types, the chart below shows the amount of Control Delay (in seconds) that makes up each category.  These are defined in the Transportation Research Board’s 2000 Highway Capacity Manual.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Level of
  Service
Intersection
  with
    Traffic Signal
Intersection
  with
    Stop Signs
A ≤10
  sec
≤10
  sec
B 10-20
  sec
10-15
  sec
C 20-35
  sec
15-25
  sec
D 35-55
  sec
25-35
  sec
E 55-80
  sec
35-50
  sec
F >80
  sec
≥50
  sec

Note – the chart is different for traffic signals and stop signs.  This reflects people having more patience sitting at a traffic signal than a stop sign.  To date, there are no Level of Service standards for roundabouts.  Most traffic engineers fudge this by applying the delay table for stop signs to the amount of delay calculated by software programs such as VISSIM, Rodel, or Arcady.  It is not an apples to apples comparison.  Hopefully the next edition of the Highway Capacity Manual will address Level of Service at roundabouts.

  • I have a question about in what situation would a car have to wait on a stop sign for over 50 secs? Is it because there are a lot of pedestrians crossing at one time or that an accident occurred? In these cases, will these situations have an impact on the grade even though they might be a one time thing? What happens if during one part of the day, there a many pedestrians passing by causing a delay of over 50 secs in a stop sign intersection, but other times it was mostly less than the 50 sec. How will this intersection be graded?

    • Chris says:

      Alvin,

      Here in Omaha I have waited at an intersection with stop signs for over 1 minute. It is normally when people are returning home in the evening and the intersection is very backed up – there is a line of cars waiting in all four directions. I am not sure how the congestion at different times of the day would affect the LOS grade, but it is a good question. Do they base it on peak hours (rush hour)?

      • Mike Spack says:

        LOS for an intersection is typically measured for a single hour and represents the average delay over that hour. In major cities, it is common for a driver waiting to turn onto a major street (with lots of traffic in the peak hour) to wait for a long time. For our studies, we will typically look beyond the LOS and use the vehicle queues (number of cars waiting at the stop sign) to determine when improvements may be needed.

        For non-peak hours, with lower traffic volumes, the LOS will be better as less time is necessary for a driver at a stop sign to find a gap. That’s why transportation studies will typically evaluate the peak hours. If operations are acceptable during the peak hours, they will also be acceptable during non-peak hours.

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