Taking apart Mayor White's spinning of red-light-camera study
There have been several follow-ups to Bradley Olson's story on the findings of Houston's long overdue red-light-camera study.
TheNewspaper.com came up with a couple of interesting points after looking the data:
The Rice-TTI study compared 24 months of pre-installation data to between 13 and 21 months of post-installation data for each of the five groups. According to TheNewspaper's analysis of overall accident data found in the appendices, the average number of monthly collisions went from an average of 15.4 collisions per month in the two years prior to camera enforcement to 58.3 accidents per month in the post-installation period. Although this figure is not reported in the study itself, the general fact is briefly acknowledged.
"The absolute number of collisions at camera-monitored intersection approaches is not decreasing," the study admitted.
To achieve the appearance of success, the study divided red light camera intersections into "non-monitored" approaches -- the directions of travel at the intersection where the red light camera is not looking -- and the "monitored" approaches where ticketing took place. There was a 132 percent increase in collisions at the non-monitored approaches of the intersection where red light cameras were installed and a non-significant 9 percent increase at the monitored approaches. The study treated these increases in both rear end and T-bone collisions as unrelated to the red light camera as long as the accident happened outside of the camera's view.
The study concluded that because the accidents went up at the non-monitored approaches of red light camera intersections, but effectively stayed the same at the monitored approaches, that the red light cameras were responsible for the "benefit" (a smaller increase) at one part of the red light camera intersection, but not the increase in acccidents at the other. This line of thought would suggest that the increased accidents at the non-monitored approaches of red light camera intersections reflected an increase in accidents at the other city intersections that had no red light cameras at all. The study admits this implication is untrue.
The second point: The data from the Houston study is at odds with the data from the statewide red-light-camera study that purported to show red-light cameras reduced accidents:
Increases in Houston collisions documented by Rice-TTI mysteriously became decreases in collisions in the TxDOT-TTI report, as follows:
Monroe at Gulf Freeway East Service Road: a 913% increase became a 41.7% decrease
Hollister at Northwest Freeway: a 747% increase became a 60.5% decrease
FM1960 West at Tomball Parkway: a 307% increase became a 44% decrease
Richmond at Dunvale: a 103% increase became no change
South Sam Houston Freeway at Telephone Road: a 164% increase became a 19.3% decrease
East Freeway North Service Road at Normandy: a 52% increase became a 25% decrease
North Freeway West Service Road at West Rankin Road: a 18% increase became a 32.7% decrease
Then Bradley Olson posted a follow-up story on Chron.com suggesting the Houston study was flawed, which prompted Scott Henson, who blogs at Grits for Breakfast, to express his skepticism:
While not long ago, the Chronicle touted the results of a much less rigorous study uncritically, in this case, where the results do not support red light cameras, the whole story was centered around claiming the research methodology was flawed. Writes Bradley Olson:
Because red-light cameras are known to have a spillover effect — meaning that they have been shown to impact the number of accidents at intersections where there are no cameras — robust examinations of camera programs always compare crash data with that in other cities.
It's what statisticians call a control group. Unless the study authors compare crashes at the 50 intersections where red-light cameras have been installed with other intersections in which they have not been — preferably in other cities — no conclusions can be drawn from it.
This is utter gobbledy gook, not a legitimate statistical analysis! Red light cameras create their OWN control group when researchers monitor accidents at the intersection BEFORE and AFTER enforcement begins.
That was the flaw I identified in the Texas Department of Transportation study published in December - in the vast majority of intersections they studied, data was not gathered before cameras were installed to compare them. In the Houston study, they had that data going back several years, so IMO the results are actually much more robust and probative than TXDOT's.
[snip]
The reporter also claims, bizarrely, that, "At a ratio of 10 to 1, study after study on the effect of red-light cameras ... have found that they drastically reduce crashes." That's simply false on its face, which further leads me to think Olson's sources were feeding him a line of bull. As I wrote in reaction to the TXDOT study:
in Lubbock red light cameras were discontinued after accidents overall increased 52% at intersections with cameras. Similarly, the state of Virginia eliminated their use after studies in every city using the devices found the number of accidents increased. In other jurisdictions, studies have found reductions in right-angle accidents but nearly equal increases in rear-end collisions, including in injury accidents.
The Houston figures jibe closely with those results, so I don't see why Olson or Mayor White are so surprised that accidents increased in Houston, too. That's what happens when officials prioritize revenue generation over public safety.
And finally, a letter in today's Chronicle, points out that most red-light-camera violations occur in the first two seconds after the light has turned red:
These are not very likely to be the ones that cause serious accidents, but they certainly are the source of a huge amount of revenue being raised by the city. Heavily penalizing these early violations does almost nothing to reduce the accident rate because most red-light-running accidents are caused by drivers who are oblivious to the red light. They run the light much later than two seconds into the red, and they are usually impaired or oblivious to the danger they face.
And a California study showed that three-fourths of all red-light-camera violations occurred in the first second after a light turned red.
The point?
Lengthening yellow light times will do more to reduce red light running and accidents than cameras ever will.
Posted by Anne Linehan @ 01/04/09 03:32 PM | Print |
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