Traffic rotaries and other speed bumps

Today, epoxy-clad reinforcing steel doesn't rust and self-destruct in the bridge concrete, but some drivers still assault each other at the lane merge and at the traffic circle, places where, more than the straight and open road, designers must assume some modicum of predictable inter-driver manners. It isn't always there.

Highway engineers have done better when designing around driver-self-interest instinct, which explains why, while Keats was writing his anti-auto polemic, the site planners of the New Seabury residential development on Cape Cod deliberately flouted standard practice and made the roads narrow, curvy, and bumpy: to keep speeds down. Today it's called the design-speed principle, and it's based on the fairly reliable fact that drivers won't do 70 where they can't see stopping distance ahead. In contrast, standard practice (on the Interstates, for example), calls for design speeds well in excess of 100 and government then hires ticket-issuers to profit from punishing the designed-for behavior.

Highway designers used to be more sanguine about driver manners than they are today: thus, when I first saw a state-of-the-art auto road built in the '20's on the Eastern Shore of Maryland, I understood that the design intent for the paved concrete center lane, flanked on each side by a gravel lane, was that drivers would courteously move off the higher-speed surface when they saw a car coming. It proved to be overly optimistic. Thirty years later, in Massachusetts, the gravel lanes were concretized, but it still didn't work: the center lane then was called "the suicide lane".

Today, some of the Interstate on-ramps display "yield" signage and some don't, and in some parts of the Heartland right-lane drivers move over, signage or not, to accommodate entrants, while coastal-State and urban drivers mostly don't. (Historical note: in 1913 Wisconsin adopted a yield-to-right rule, giving legal priority to the on-ramp charioteers. It hasn't survived.)

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