Level of Service Introduction – How Spack Consulting Conveys Results
Level of Service, or LOS, is a qualitative description used by traffic engineers to communicate how good or bad traffic operations are on a corridor, intersection, interchange, or other roadway conflict area like a weaving section. The latest Highway Capacity Manual includes data and procedures to develop LOS for bicycles, pedestrians, and multi-modal corridors. Just like most schools, LOS uses letter grades to define operations. LOS A represents the best operations with free-flowing traffic and little to no congestion. LOS F, on the other hand, represents failing operations, where congestion is heavy with excessive delays. LOS E is considered “at capacity”, where the supply of traffic meets the capacity of the infrastructure.
The idea behind the LOS rating concept is two-fold: First to provide a method to quantify congestion, allowing comparisons between corridors, improvement options, etc. Second to provide an easy to understand method of conveying results to the general public. This concept has been adopted across the United States and in many other places around the world.
While the LOS grades succeed in providing a quantifiable measure in an understandable format, they are not without some issues. At Spack Consulting, the two primary issues we have seen in regard to LOS ratings are:
+ Relaying what is considered acceptable to the public.
+ Difficulty in relying on LOS results in determining mitigation for side-street stop controlled intersections.
Let’s tackle these one at a time.
If the comparison is to a school grade, then all infrastructure should be striving to reach LOS A at all times of the day. However, we know that is not realistic nor cost effective. Instead, most agencies designate a lower grade as acceptable. Typically, LOS D, like school grades, becomes the acceptable grade. To which, in some cases, we have been asked why we shouldn’t be mitigating to LOS A. To present the information in a manner better understood, we have shifted our reports to using graphics. The implications of a development or roadway change are clearly indicated side-by-side and the “pass-fail” line is also presented. Here’s an example below.
This type of display works well for traffic signals, roundabouts, and all-way stop controlled intersections where every approach will stop at some point. For side-street stop control, the major road continues without stopping, which by definition is LOS A. Given the major street can have significantly more traffic than the side street, the overall intersection LOS is often skewed and not a reliable indicator of operations. However, the side street LOS may also fail to provide the whole story. During peak hours, it is not uncommon for a driver to wait one or two minutes before being able to turn onto the major street. That delay would be reported as LOS F, potentially indicating mitigation is necessary. However, if only one driver makes that movement, mitigation would not be justified despite the poor LOS.
Specifically for our side-street stop controlled intersections, we have started using the 95th percentile vehicle queue as the appropriate metric. Queues between five and ten vehicles (or more) may suggest mitigation in the form of additional lanes, geometric improvements, or other measures may be needed. This can be seen in the graphic below. Again we provide the scenarios side-by-side with a clear line of “pass-fail.” The range of acceptable-unacceptable is unfortunately not based on qualitative or extensive data research. Instead, it is based upon our own observations and judgments about when we feel mitigation could be justified.
We think these methods better conveys the results to the public and, sometimes more importantly, the public officials who may be voting on a project or development. How are you conveying your study results?