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Saturday, October 16, 2010

Access Control

Access control is the phenomenon where traffic engineers close or limit the number of curb cuts providing access to a roadway, typically a high-capacity facility such as an arterial.

For automotive users, the benefits are less certain. In traffic movement, the ideal is 'laminar flow', where you have a steady stream of cars all moving at the same velocity, until you reach 'maximum saturation' of the lane, where adding an additional car causes the car behind it to slow down, in order to maintain a safe following distance. This in turn causes the second following car to slow down, and so on, rippling back down the line of cars.

Traffic engineers apply an equation to calculate delay based on a number of factors, most notably # of lanes and facility type. Facility type covers a multitude of geometric design factors--curvature, slope, turning radius, speed, etc.

At some point, someone realized that roads with a large number of access points did not move traffic as well as those without. On this basis, it seems logical to reduce access points, because each access point represents a disruption in laminar flow, slowing the speed of traffic. And so traffic engineers began to close access points, to improve traffic flow.

But this is treating the symptom, not the cause. Why does the road have so many access points? Because there are that many business accessing the road?

But closing access points is treating the symptom, not the cause. Acme Widget Co. has 300 employees, and closes at 5 PM, five days a week. Their parking lot has three access points onto the street. 150 cars leave using each exit. Cut it down to one access point, and you have 450 cars leaving via that one way. You have fewer disturbance points to laminar flow, but bigger disturbances.