Heat is a road condition
Weather gets written down on a crash report when it changes the road surface. Rain, ice, standing water, fog. Heat leaves the surface looking exactly the way it looked in April, which is most of the reason it gets written down as nothing. What a hot afternoon changes is the driver and the tire, and neither of those is a road condition in the sense the form means.
The research on the driver half is better than people assume. A team in Catalonia linked eleven years of crash records to daily readings from 66 weather stations and published the result in Environmental Health Perspectives. Across 118,489 crashes in the warm half of the year, the estimated risk of a crash rose 2.9 percent on heat wave days. Narrowed to crashes involving driver performance factors, which the study defines as distraction, driver error, fatigue or sleepiness, the increase was 7.7 percent. For that group the risk climbed 1.1 percent for every additional degree Celsius of daily maximum temperature. One percent a degree is a small number. It applies to every degree, on every day, across a season that in some places runs four months.
What the heat does to the tire
The vehicle half is less argued about. NHTSA's report on tire-related factors in the pre-crash phase states the mechanism in one sentence: under extreme temperatures, tires are vulnerable to degradation, significant loss of pressure, additional flexing and stress on the sidewalls, and those conditions can end in a failure or a blowout. Low pressure compounds it, because a tire running below the pressure it was built for generates its own heat and that heat accelerates the wear. The same report found that tires underinflated by more than 25 percent were about three times as likely to be cited as a critical event before a crash.
That report also contains a useful failure. It sorted crashes into hot months, cold months and mild months, and found that 5.5 percent of vehicles on the road from July through September had a tire problem before the crash, against 4.6 percent in the mild months. The difference was not statistically significant, and the authors say why in a single line: the categorization did not take temperature variation by state or location into account. A July in Austin and a July in Duluth went into the same column. A national data set that does not know where the car was cannot see a heat effect even in the places where there is one.
The length of the hot season in Austin
Austin has the temperature and it has the duration. The National Weather Service office covering the region puts the normal high at 90 degrees by May 26, holding above 90 until September 23. That is four months. The record high is 112. In 2011 the city recorded 90 days at or above 100 degrees.
The figure worth sitting with is a quieter one. Across the full period of record, which starts in 1897, the average last 100-degree day of the year falls on August 21. Across the most recent thirty years it falls on August 30. Nine days were added to the tail of the hot season inside a single working career, and they land in the part of the calendar when a tire has already had a full summer behind it. None of those numbers describe the surface a tire is actually touching. Air temperature is measured in shade, at about five feet, over grass, conditions that almost no outdoor work happens under, and neither does much driving. A paved lot in direct sun runs well above the number on the phone, and so does the inside of a closed cab.
Who the question belongs to
One more finding in that NHTSA report changes how these events end up on paper. Half of the tire-related crashes were single-vehicle crashes. Among the crashes where tires were never cited, the figure was 31 percent. A single-vehicle crash has nobody else in it. It gets written up as a driver who left the road, and the tire that came apart thirty feet earlier becomes a detail inside that description instead of the subject of it.
The people who find out what a crash report left out are the ones who have to reconstruct it afterward, which is why the distinction lands on them first. An Austin car accident lawyer who handles these claims sorts a hot-weather crash into one of two piles before anything else. A tread separation on hot pavement is a maintenance question, and it runs toward service records, the age of the tire and whether a pressure warning had been showing for a month. A driver who goes down in a delivery van whose cab has been sitting at 108 degrees is an employer question, and it runs toward the route, the length of the shift and whether the air conditioning worked. The crash report has one line for both.
The record-keeping fix is dull and cheap. The temperature at a given place on a given day is a number the weather service already measures and archives, and putting it next to the crash record joins two files that both exist. Until somebody does it, the hot months keep producing crashes that get counted as ordinary ones, and the only people asking what the temperature was that afternoon are asking on behalf of somebody who is already hurt.