Anyway, the reason I balked a bit at the drum-buffer-rope label is that most descriptions emphasize a single constraint as the drum, in contrast to the situation in a balanced production line, where the current constraint bounces around less predictably — which is how I see the traffic situation.

The article doesn’t go into the details but does say that “Lights are coordinated with every neighboring light, such that one light alerts the next, Hey, heavy load coming through.”

From that statement I’m inferring that the drum is not any one light or even every light. The system may not be that smart at all, but it certainly could be tied to a natural bottleneck as you suggest and use that as the drum, or the drum could be non-physical, as is the time of a strategic resource in project management. This might look like optimizing throughput in a section or to some “safe maximum” rate.

My guess is that by connecting the in and out sensors of a series of lights together that they’ve created an implicit drum. Bohm meets Goldratt!

Another possibly interesting system might use something similar to a TOC inventory dollar days calculation — incorporating time spent in some road section before a tunnel or bridge and the toll charge for that vehicle. Public transportation and commuter vehicles that might not pay tolls could have a higher pseudo-toll for this calculation, enabling road managers and planners an easy mechanism for lane assignments, signal gating, and such, depending upon their goals (such as getting private cars in and out of suburbia or encouraging mass transit.)

In this case the rope is the communications between traffic signals, the buffer is the “right amount of time” used as the delay and the drum is the rate at which the road can safely handle the traffic demand.

Put another way, the traditional traffic management approach is similar to Lean Manufacturing’s balanced production lines, only with the balance keyed to the clock time to meet average demand. Unfortunately, due to all kinds of variables including accidents, weather, driver behavior and statistical variation, a bottleneck (traffic jam) can easily occur. This bottleneck can occur anywhere so it cannot be managed in advance and the system cannot recover until the load decreases.

Accidents, weather, etc. still affect the new traffic system in the posting but the system’s simple intelligence can restore maximum throughput far more quickly.