Reader’s Note from Temperance Blalock: Speed Cameras and Traffic Citations
Aug 9, 2010 20:32 - 6 Comments
Posted by: Cindy Cotte Griffiths
Department: Contributor Opinion,Opinion
Tags: by Temperance Blalock, Reader Notes
Department: Contributor Opinion,Opinion
Tags: by Temperance Blalock, Reader Notes
Chart by Temperance Blalock
After seeing today’s POTD: Speed Sentinel, Temperance Blalock sent along this chart and Reader’s Note:
There’s a great deal of information available on the City website regarding speed cameras and traffic citations (http://www.rockvillemd.gov/police/speedcameras.htm). The “raw data” for many of the fixed-pole camera locations is presented, and I thought I’d play around with it and see what underlying trends I might be able to find. The “average speed” in itself wasn’t really helpful, other than to show broad trends of speed decreasing. I would have much rather seen the “average speed” only for the speeding ticket recipients.
The total numbers of vehicle passes and citations issued, by themselves, didn’t really show anything. However, when I divided the number of vehicle passes by citations issued, to get a number for “citations issued per vehicle pass”, I found an interesting pattern.
The “spike” in Fall 2009 was repeated almost uniformly in the five locations that I plotted. What do you suppose that this could “mean”? Is there a coincidence that it occurred shortly before the City election? Or perhaps the cameras were all recalibrated around that time? Or could the citizens of Rockville have suddenly been spontaneously seized with the urge to drive recklessly?
by Temperance Blalock
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Thanks, Temperance. Can you clarify what the Y axis is?
The Y axis is actually “vehicle passes per citation issued”. So, for example, the data for 500 W Montgomery Ave eastbound is found here. In May 2009 there were 317198 vehicle passes, and 348 citations were issued, for a rate of 911. In October 2009 there were 312172 vehicle passes, but only 127 citations were issued, for a rate of 2458.
The number ‘vehicle passes per citation issues’ is a bit misleading because an increase in vehicle passes per citation issued translates into fewer citations, not more, right? Someone who doesn’t know much about statistics might look at your graph and think more citations were issued in October, rather than fewer. I would have switch up the calculation as follows:
348/317198*10,000 = 11 citations for every 10,000 vehicles passed in May
127/312172*10,000 = 4 citations for every 10,000 vehicles passed in October
Yes Stephanie that’s what I thought after I read the introductory paragraph….so actually October had, for some reason, a lower rate of citation. For that matter, why would citations per vehicle pass be so much more frequent in February 2009 on W Montgomery?….it seems to me the best explanation for these anomalies may be calibrations to the equipment that triggered a change in citiation frequency? The rates of citations, if the equipment functions consistently, shouldn’t result in such outlier fluctations for something as broadly predictable as people’s driving behavior, should it?
I’d be curious to see the data for 2010 as the rate of citations appears to be on the decline into 2010.
I wonder if the smart phone applications that alert drivers to the locations of speed cameras might have something to do with it?
I am not a statistician, so I am certainly open to discussion of how this data can be interpreted in a variety of ways. However, there’s no doubt that there are some odd patterns, and what struck me was that the trends seemed to be parallel among the different sites.
I believe that the Feb 2009 numbers for W Montgomery may have been posted wrong. Jan 2009 had 239929 vehicle passes, March 2009 had 261383. The posted number for Feb (21012) is actually probably ten times that.
There are many comments about W. Montg. and regulating speed but no one seems to be of help with the speeding on Maryland Ave. Havea 25mph sign up and a weight restriction on trucks. I watch traffic going up to 50mph and tractor trailers using Md.Ave. Been told that when police are not too busy they would polce . But when I go see my relatives at Rockville Cemetary in the middle of the day there are police cars parked (Napping??) I requested the 25mph sign going West be placed nearer the stoplight at Md. and Fleet so they put it at 5 Md under a tree and by the time they get to my driveway (where I have to back out ) they are really speeding.. My father built my house in 1947 and it was a deadend street. Thanks for once again letting me blow off steam. MGA