Roadway "capacity" is the traffic volume above which "congestion" becomes "unacceptable"

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We all have some experience with the concepts of transportation "capacity" and "congestion". Here, I'd like to establish a level of understanding that will enable us to evaluate the proposed (planned?) configuration changes to the NM502 corridor in Los Alamos, in comparison with the status quo.

Congestion is what happens when transportation fails. Transportation can fail in various ways and at a range of intensity levels and frequencies. Adverse effects of traffic congestion include

  • traveler/driver fatigue
  • increased travel time
  • traffic queues
  • increased accident rates
  • traffic jams
  • road rage

Roadway capacity can be calculated as an engineering figure of merit. Calculations of capacity vary in sophistication. We'll look at a simple method.

No matter how complex the models or simulations employed, an element of judgment is involved in interpreting the results: what is the acceptable intensity and frequency of adverse effects? Unfortunately, individuals have widely differing levels of patience or tolerance for transportation failures. This makes it difficult to arrive at a consensus of how to define a reasonable threshold for unacceptable dysfunction.

Once a roadway capacity has been determined, we define the "utilization" of the roadway as the traffic volume divided by the capacity (in per cent). This quantity is also called the "volume to capacity ratio" and the "saturation" in traffic engineering reports. A roadway with the traffic volume equal to its capacity has 100% utilization, or a saturation of 1. Most drivers would agree that the road is severely congested under this condition.

Next, we'll try to illustrate these concepts at a level that is crystal clear and helpful in evaluating the changes proposed in the NM502 Corridor Study by developing a simple model for roadway capacity.

If you have a moment for humor, or would like to see a few traffic nightmares, take a look at photos here and here. (These links take you off site, and you'll have to use your browser's "Back" button to rejoin the NM502 discussion.) Also, you might wish to look at the ultimate "jam-free" traffic plan.

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