traffic terminology

Here's a brief guide to understanding some basic terms used by traffic and transportation engineers:

AADT

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

LOS (Level Of Service)

LOS, or Level Of Service, is a measure for driver delay at an intersection. It ranges from LOS "A" (hardly any delay) to LOS "F" (the driver waits at the intersection for more than one red light cycle).

One of the troubles with deciding the number of lanes for an intersection based on LOS is that it only considers the experience of those traveling in automobiles while ignoring the experience of pedestrians, transit users or bicycle riders, and neglecting to consider the inter-related effects that transportation systems have on issues like quality of the pedestrian realm, the economic development potential of a place, or the relative difficulty or ease of crossing a street for those not on cars.

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 ..."

It is also important to note that the LOS figures that are cited in justifying road expansions are usually 20 year projections covering the peak 15 minutes of traffic in a day - in other words, we expand roads today so that they meet LOS requirements for the busiest 15 minutes of projected traffic for a day twenty years from now! Again from Khisty: "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 an actual traffic decrease of 12%:

Unfortunately, the tendency is to treat traffic projections as if they were some kind of fact set in stone, rather than as the highly simplified projections and assumptions that they are. As Project for Public Spaces said in their assessment of the Lake Street Reconstruction Project: "Future volumes are a warning, not a mandate, and with different policies the city and county can institute a multi-modal goal for Lake Street."