As the Road Worrier reports today, red-light cameras installed at 11 intersections in 2003 are credited with an 83 percent drop in right-angle "T-bone" crashes. Raleigh counted crashes four years before, and four years after the cameras went into operation.
Here are the stats at each intersection.
Brentwood at New Hope Church: 31 T-bone crashes in the 4 years before cameras were installed, 3 in the 4 years after, a 90% reduction
Dawson at South: 45 before, 16 after, 64% reduction
Dawson at Morgan: 40 before, 1 after, 98% reduction
Six Forks at Rowan: 34 before, 2 after, 94% reduction
Hillsborough at Dixie / Friendly: 19 before, 2 after, 89% reduction
New Hope / Millbrook at Capital: 23 before, 4 after, 83% reduction
Six Forks at Dartmouth: 17 before, 4 after, 76% reduction
Capital at Highwoods: 32 before, 17 after, 47% reduction
Peace at West: 31 before, 6 after, 81% reduction
New Bern at Tarboro: 23 before, 1 after, 96% reduction
Rock Quarry at Cross Link / Proctor: 42 before, 2 after, 95% reduction
TOTAL: 337 crashes before, 58 crashes after, 83% reduction
source: City of Raleigh
Bruce Siceloff reports on traffic and transportation. A News & Observer reporter, editor and blogger since 1976, he took over the
Comments
Comparison?
Tue, 08/26/2008 - 16:17 — Steve (not verified)I might be missing where it's listed, but this information is meaningless to me without a comparison of 11 (or some other number) of similar other intersections where cameras WEREN'T used, with the same data from 1999 and 2007. That's when these numbers will be meaningful -- you've got to have something to compare these data to!
Before and after
Wed, 08/27/2008 - 12:18 — BruceSiceloff (author)Steve -- It seems to me that they're making the appropriate comparison:
Counting crashes for four years at specific intersections, then installing red light cameras, then checking to see how many crashes there were over the next four years at the same intersections.
How did the number change after the camera was installed? It's like, say, counting traffic injuries before and after a state changes its seat belt etc laws.
That's not how it works!
Wed, 08/27/2008 - 19:00 — Steve (not verified)When you make a change -- putting up cameras at certain intersections -- you need to compare the data to data from intersections that DIDN'T get cameras added. Only then can you study the effect of the change (the dependent variable is the number of crashes, the independent is the cameras). The idea that Raleigh put up cameras at some intersections may have made drivers across the city more aware of every intersection and therefore there was a decrease in crashes all over. Get the data from the "un-camera-ed" intersections and compare it to the data you have and then you can say something about the cameras. (Ask some statistician if you don't see this.)
Statistics
Fri, 08/29/2008 - 09:24 — BruceSiceloff (author)Thanks for the suggestion but I'm not going to pursue this further, at this point. I don't take the city's numbers as a conclusive engineering study that proves anything. But it is impressive evidence -- after all, the drop in crash counts was huge -- that the presence of cameras has prompted drivers to take more care at those intersections.
If citywide statistics did indeed show a big drop in T-bone crashes at non-camera intersections as well, what would we infer from that?
Perhaps that Raleigh drivers have improved everywhere of their own volition, without regard to cameras, and that cameras do not deserve any credit after all.
Or as Steve suggests, the opposite: that the beneficial influence of cameras has spread to non-camera intersections across the city.
Or neither of the above.
I shake my head.
Fri, 08/29/2008 - 18:58 — Steve (not verified)I don't care one way or another, but then, I'm not paid to. I'm just surprised that you put such value in statistics that seem meaningful to you (83%!) but aren't interested in the comparison statistics that would actually tell you how to interpret that number. 83% could actually be higher, much higher, lower, much lower or the same as other intersections without cameras, but we'll never know.
Statisticians may quibble
Fri, 08/29/2008 - 09:52 — Dadof3The change in the before and after strikes me as meaningful. Yes, a control group is helpful but more so in less dramatic circumstances. I also like the fact that our police can move on to more substantial tasks than ticketing red-light scofflaws. There seems to be security and personal liberty issues involved with traffic cameras but we are already to glib about our responsibilities as drivers.
I've been tagged by one of those cameras
Tue, 08/26/2008 - 07:52 — Dadof3of course I didn't like it, but it's hard to argue with a 83% reduction in accidents.