Monday, October 27, 2014

Random Number Generator

Randomness is a very important tool in cryptography. Not so much for speed limits.

But that appears to be what we have with our new variable speed limit signs on top end 285.

If you enter 285 at Ashford Dunwoody heading towards Spaghetti Junction you will pass more than four of these signs, spaced about an eighth of a mile apart, before you get to the Doraville Speed Trap. One recent morning the sign at Chamblee Dunwoody read 55 MPH, the next was a slowdown to 45 MPH and the very next one (near the GM plant) was 65 MPH.

Seems a bit odd though it was clear that drivers were no more paying attention to these signs than they did the previous, low-tech, low-cost ones. From the very beginning those who drive this stretch of interstate knew just how silly these signs are doomed to be. Drivers ignoring any signage undermines the key assumption of the ill-fated plan: that drivers will observe the signage and obey the currently posted speeds.

We've been told that the speed is set according to "where you are going" rather than to the immediate conditions near the sign. This in no way explains the 55-45-65 MPH sequence. We've also been told this is about safety, about wreck avoidance and prevention. Of course any wreck on 285 has the knock-on effect of bringing traffic to a standstill so safety and congestion management are inseparable. Still, this is nonsense.

Perhaps a more likely explanation of this silliness is that the agency controlling  these signs is seeking an IGA with local law enforcement putting in place a revenue sharing plan. Until then speeds will be set to minimize speeding tickets but once the agreement is in place then speeds will be manipulated according to the revenue needs of these agencies. Perhaps they will even set the signs at the shoulder to a different speed than the one right across at the barrier wall (they are not always showing the same speed) so the Local LEOs have discretion on how much they get for which ticket (and ticketed party).

Or perhaps it is just another case of grant-grubbing out of control.