I recently suggested the Cheyenne city council killed 9 people in 2005 by lowering the speed limit in 2004, with an invitation to think about why the suggestion was misleading.
The big lie, which you should be thinking about as you read the latest round of “fatalities tripled!” headlines, is use of only one year before and one year after.
Let’s look at more data, fatal accidents from 2002-2006: 23, 6, 22, 13, 16. There were 29 before and 29 after. The “doubling” is no change at all.
In statistical terminology car crash rates are “overdispersed.” If they were well-behaved random events the year-to-year variation would be about the square root of the annual number, so about 4 in Laramie County or 170 in the whole country. The year-to-year variation in the series above is about 7. I think of car crashes as “more random than a random number.”
That more-than-random variation provides an opportunity to “cherry pick” only favorable evidence. You can publish a subset of data that fits your theory, knowing that most people won’t look for the truth. I could have said “reduced from 23 in 2002 to 16 in 2006.”
There were other problems with my claim, but they were not misdirection. They were limitations in the data I could easily find.
I only gave figures for Laramie County as a whole. Wyoming’s reports break down rates by county. Cheyenne is most of the county, but for all we know the deaths were outside city limits.
Within the city, the speed limit change only affected some roads. For all we know, the deaths were on other roads. Here you have to be more careful with the analysis, because enforcing a reduced speed limit could have diverted traffic to other streets.
Some say the national 55 mph speed limit moved deaths from Interstates to secondary roads. Would you rather drive on a speed-trapped 55 mph Interstate or a scenic 55 mph two lane road? It’s tempting to choose the two laner, but it’s also more dangerous.
If reducing the speed limit made the Interstate safer (which I don’t believe), it could still end up killing people.
Ohio raised the speed limit for trucks several years ago to encourage truck traffic to move to the Turnpike. Truck drivers were rolling across the state on two lane roads because the speed limit was the same and there were no tolls.
This was a rare look at the big picture. Ohio is better off with trucks using highways designed for them, even if the Turnpike’s budget suffers. (I’m referring to increased maintenance cost here, not only loss of ticket revenue.)
Closer to my home, city engineers retroactively justified some stop signs by pointing to a reduction in accidents after they were posted. Nobody asked if accidents were up on the alternate route with fewer stop signs. They were looking for an excuse rather than solving a problem.
In many cases it’s only a few minutes work to discredit the latest headline, but there aren’t enough scientifically literate reporters. For understandable reasons, they tend to cover science and medicine or sports and politics.
Since reporters aren’t using their heads covering transportation safety, you have to use yours.
The opinions expressed in this post belong to the author and do not necessarily represent those of the National Motorists Association or the NMA Foundation. This content is for informational purposes and is not intended as legal advice. No representations are made regarding the accuracy of this post or the included links.