traffic terminology

They like to use terms that they never define and that nobody understands to mask all the auto-centered assumptions that underlie traffic projections and recommendations - so here is a quick guide to understand some of their basic terminology:

AADT

Average Annual Daily Traffic, the average number of vehicles that use a given road per day.

LOS

Level-Of-Service, ranges from LOS "A"to LOS "F". According to "Transportation Engineering - An Introduction", by C.J. Khisty:

"LOS for signalized intersections is defined in terms of delay. Delay is a measure of driver discomfort, frustration, fuel consumption and lost travel time ..."

"LOS criteria are stated in terms of the average stopped delay per vehicle for a 15-minute analysis period ..."

traffic projections

Traffic projections are predictions, based on mathematical models, that attempt to quantify the number of vehicles that will travel on a given road at a future date. Like other models, their results are dependent on initial assumptions and other simplifications necessary for the modeling process (i.e. traffic growth rates, trip assigment, alternative modes of transportation, changes in land use patterns, etc.).

Traffic models are notoriously unreliable for predicting future travel patterns on complex urban grids. The example below compares the forecast 2000 Lake Street traffic volumes (projected in 1995) with the actual vehicle counts measured in 2000. As you can see, the actual traffic levels vary significantly from those predicted; in the case of the stretch from Chicago to Cedar, what was predicted to be a traffic increase of 23% was instead a decrease of 12%: