When you fill up your fuel tank at a gas station, have you ever seen a Department of Transportation official measuring the wheelbase of your car? Neither have I, but the Federal Highway Administration’s Highway Statistics, table VM-1, reports the total amount of fuel used, to the nearest 1,000 gallons, by vehicle type. Those vehicle types include buses, motorcycles, large trucks, and short- and long-wheelbase automobiles, with the division being wheelbases of 121 inches.
The government has a pretty good idea of how many miles Americans drive each year based on 5,000 traffic counters all across the country. To be honest, the traffic counters are really only on major roads, so the miles driven on local roads are just estimates. Even on the major roads, I doubt the traffic counters are good enough to detect the difference between vehicles whose wheelbases are shorter or longer than 121 inches.
Even more difficult is determining how many gallons of fuel are used by each type of vehicle. As near as I can tell, these numbers are based on a model and I’m not sure some of the assumptions in the model are valid. As I’ve noted before, I’m particularly suspicious of the bus numbers, but I have to wonder about motorcycle and short- and long-wheelbase autos as well.
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In 2009, the division used for automobiles changed from “cars and light trucks” (meaning pickups, full-sized vans, and SUVs) to “short- and long-wheelbase vehicles.” Some people, including Michael Sivak (whose work was discussed here yesterday) continue to apply the terms cars and light trucks to these categories. But the change led the average fuel economy for cars/short-wheelbase vehicles to increase from 22.6 to 23.8 miles per gallon, while light trucks/long-wheelbase dropped from 18.1 to 17.4 miles per gallon. This presumably happened because crossover vehicles such as Subaru Outbacks, which get more than 30 mpg, switched from the light trucks to the short wheelbase category.
A lot of people rely on these numbers, but the more they are broken down into categories, the less reliable they are. While miles per gallon of the total motor vehicle fleet (which were the numbers I used yesterday) are fairly reliable, miles per gallon of short- and long-wheelbase vehicles as well as other categories are more questionable.
This unreliability of highway data reminds me of a talk on traffic safety I heard by a traffic engineer about 20 years ago. He had a slide showing something to the effect (I forget the exact numbers) “if highway speeds can be reduced from 78.26 mph to 72.58 mph traffic fatalities will be reduced by ….” I asked him what the error on these numbers was as I didn’t believe that they were accurate to two decimal places. With a straight face he said that traffic safety was a discipline that had originated from civil engineering, and civil engineers don’t do statistics.
So much of our supposed traffic safety numbers may not be statistically valid!
Explain to me in detail how if China get’s to build 250 Gigawatts of coal, 100,000 miles of pipelines and 60,000 miles of automotive highways will have any positive effect on emissions, Just because Biden put us on the Paris Accords and cancelled one pipeline.
Presidential politics and science are mutually exclusive endeavors.