Two different measures are used to rate the flow of traffic on each highway link, depending on the type of highway. For uninterrupted-flow highways such as freeways and toll roads, the density of vehicles on each link is first determined by taking counts from the aerial photographs; the units of density are passenger cars per lane per mile, or pcplpm. (Trucks and buses are converted to passenger-car equivalents for this calculation.) Once average density values have been determined for both directions of each link, they are converted to level-of-service (or LOS) ratings based on the following conversion table:
|Density (pcplpm)||LOS||Traffic description|
|0 to 11||A||Very light traffic|
|12 to 18||B||Light traffic|
|19 to 26||C||Moderate traffic|
|27 to 35||D||Moderate to heavy traffic without significant slowing|
|36 to 45||E||Heavy traffic with minor slowing|
|>45||F||Congested traffic involving slowing and stopping|
Use of the LOS scale makes it easy for non-technical persons to appreciate the nature of the flow on each highway link. For this reason, LOS is promoted in the 2000 Highway Capacity Manual (HCM 2000) as a recommended way to convey the nature of traffic flow to stakeholders and decision-makers.
HCM 2000 also defines a way to determine LOS for the other primary type of facility, interrupted-flow highways (characterized by signalized intersections). However, for signalized highways density is not a suitable measure because interruptions caused by traffic signals serve to cluster vehicles and release them in groups (or 'platoons'); this causes density measurements vary wide when conditions are fundamentally the same. Instead, HCM 2000 uses average travel time as the primary basis for defining LOS on this type of highway. This type of data is often collected by driving instrumented cars through the traffic stream, called the 'floating car technique'.
However, average travel time cannot be cost-effectively measured from photographs taken from fast-moving aircraft. Instead, a surrogate methodology for approximating arterial LOS has been developed by Skycomp, Inc. This surrogate methodology uses the presence and population of vehicle platoons, and the degree of queuing at signalized intersections, to define the six surrogate LOS ratings:
|Surrogate LOS||Nature of traffic flow|
|A||Very light traffic flow;|
|B||Light flow without clearly defined platoons;|
|C||Platoons less than 15 vehicles per lane (vpl);|
|D||Platoons between15 and 25 vpl;|
|E||Platoons > 25 vpl; queue > 20 vpl at no more than 2 signals;|
|F||Three or more signal queues >20 vpl; or one severely congested signal (>40 vpl)|
(Note: these values are underlined to emphasize that they are surrogate LOS measures, not true HCM 2000 LOS measures.)
Performance-Rating Tables (Freeways)
For uninterrupted-flow facilities, the ratings are density-based level-of-service designations 'A' through 'F', as defined in the 2000 Highway Capacity Manual. A summary of density based level-of-service is provided below (a more detailed discussion of level-of-service is provided here).
Performance-Rating Tables (Arterial Highways)
For interrupted-flow facilities, a surrogate level-of-service measure has been used. Developed by Skycomp for use with overlapping aerial photographs, this surrogate measure is based on platoon sizes and queuing characteristics at signalized intersections. Because this is a surrogate LOS measure, the letters 'A' through 'F' have been underlined to identify them as a surrogate LOS measure. A summary of the surrogate level-of-service is provided below (a more detailed discussion is provided here).