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:
||Traffic Flow Description|
|0 to 11
||Very light traffic|
|12 to 18
|19 to 26
|27 to 35
||Moderate to heavy traffic without significant slowing|
|36 to 45
||Heavy traffic with minor slowing|
||Congested traffic involving slowing and stopping|
By reformatting this table as a scale and adding color, it becomes more clear how the segment densities and Levels of Service translate into the colors used in the maps (the red and orange arrows) and reports.
Density values and level-of-service ratings form the basis for analysis of uninterrupted-flow highways. Graphics are also prepared which use colors to depict various levels of density values, ranging from very light traffic (green) to severely congested (red). This table serves to translate the colors used in the report graphics.
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 (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:
|Traffic Flow Description|
||Very light traffic flow|
||Light flow without clearly defined platoons|
||Platoons less than 15 vehicles per lane (vpl)|
||Platoons between 15 and 25 vpl|
||Platoons > 25 vpl, or queues > 20 vpl at no more than 2 signals|
||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.
As with the freeway level of service table, these surrogate levels of service can also be displayed on a scale with color corresponding to the colors used on the maps (the red and orange arrows) and reports.