As the investigation into the fatal crash that killed actor and all-around car guy Paul Walker continues, a new theory has surfaced that pins the blame on the plastic dots that line streets everywhere. While the news media has run with this theory, I'm extremely skeptical.
Today, well-sourced gossip website TMZ reported that Walker's family members and certain crash reconstruction experts currently believe the Porsche Carrera GT might have spun out due to Botts' dots — those tiny colored dots that mark the lanes of travel along roads. Here's what TMZ said:
Sources close to Paul's family tell TMZ ... the family has been in touch with stunt experts who have gone to the crash site and come back with what they believe is a solid theory — a speeding car that hits the plastic markers — called Botts' Dots — will hydroplane, causing the driver to lose control.
We've learned the experts have told the family ... a car going speeds approaching 90 MPH will lose traction after hitting a series of Botts' Dots. It's akin to driving on ice — and control is further compromised by the fact that the Porsche was outfitted with special racing tires that have very little tread.
Let's ignore that "special racing tires" remark, and how it's more fearmongering with the Carrera GT, a difficult-to-drive supercar that has been wrongfully made out to be a deathmobile in the last few weeks. We'll instead focus on the claim made by TMZ's unnamed sources say lay blame on how the car reacted when it drove over the road dots.
Right away, I had a hard time buying this, but it has more to do with personal experience than anything else. I have never, ever, ever heard of any crash related to road dots, and thanks to the better part of a decade I spent pre-Jalopnik covering cops for a major Texas newspaper, I've seen and written about just about every way a person can die on the roads. Alcohol? Certainly. Bad weather? Oh yes. Driver error by one or more parties? Most definitely. But deaths caused by road dots are a new one on me.
But my anecdotal experience isn't enough. After reading TMZ's report I went in search of statistics or data on crashes, fatal or otherwise, related to road dots. I could find none.
So I turned to physicist and friend of Jalopnik Dr. Stephen Granade, a man who knows far more than I ever could about traction, bodies in motion and momentum. Not surprisingly, he too was skeptical.
Here's what Dr. Granade told me, in full, emphasis mine:
Could Botts' dots have caused the Carrera GT to lose traction and, in effect, hydroplane around? Well, maybe. Cars don't slide around thanks to friction. The engine turns the tires in a circle, producing torque. The torque will spin the tires freely unless they're gripping the pavement tightly enough that the tire surface can't slide relative to the pavement. If the tires lose their grip, then they spin freely and you don't have good control of the car.
The grip is caused by friction: the force that keeps the tire from sliding across the pavement. Once your tire starts spinning you've got a problem, because the force of friction between two surfaces moving past each other is way lower than when they're at rest. If you push a packing box across carpet, it's a lot harder to start it moving than to keep it moving.
In hydroplaning, you get a sheet of water between the tire and the pavement. That thin sheet of water is slick and so the tire starts spinning. Could that have happened to the Carrera GT? If it happened, the most likely way would be for the tire to lose contact with the pavement as it rode over a dot. Since the dot is slick, the tire spins. The car loses traction, leading to the crash. But for that to happen, the dot would need to be pretty big compared to the part of the tire that touches the road.
Calculating the dots' size is the easier part. Caltrans mainly uses circular Botts' dots that have a 4" diameter, which matches what I saw in the pictures on TMZ and elsewhere. The surface area of a circle is pi * radius^2. So those 4" Botts' dots have a surface area of about 12.6 square inches.
What about the tire's surface area? Growing up, I'd always heard that a car tire's contact patch, the area of the tire that's actually gripping the road, is about hand sized. But we can get better numbers than that with science! (Or at least some math and back-of-the-envelope calculations.)
The air pressure in your tires is a measure of how much tire contact patch you need to hold up the car. If you inflate your tires to 32 pounds per square inch, then the tire can hold up a pound for every square inch of surface area it has pushing on the road. If we figure out how much weight each tire is holding up, then we'll have a good estimate of how big its contact patch is.
To figure that out, we need to know how much the car weighs, what percentage of that weight is on the front tires, and what the tire pressure is. The 2005 Carrera GT has a curb weight of 3000 lbs. There were two passengers, so let's be conservative and add another 300 lbs. The car's weight distribution is roughly 42% on its front tires and 58% on its back, so its front tires will have the smallest surface area. Finally, the car uses Michelin tires that should be inflated to 32 pounds per square inch. With all of that information, we can calculate what the tire surface area is:
Tire surface area = (weight * front distribution)/(number of tires)/(tire pressure)
Take the total weight of 3300 lbs, multiply by 42% to get the weight on the front two tires, divide by two tires, and divide by 32 pounds per square inch. You get just under 22 square inches.
That's almost twice as big as the dot! There's no good way for all of the tire to be on the dot. Since tires have some give in them, the tire would still be in contact with the road even as it was on the dot. That means that this isn't like hydroplaning: the tire's still mainly in contact with the road and benefits from the road's high friction force on the tire.
While this isn't an exact calcuation, we'd have to be way off on our tire calculation to begin to get close to the dot size. The car weight would have to be under 2,000 lbs or the tires inflated to more than 55 psi before the tire contact patch would be as small as the dot.
There are other factors that make me doubt this explanation. The way the dots are spaced, you're most likely to have only one tire on the dots at any time, leaving three perfectly fine tires. The Carrera GT has a traction control system to deal with this kind of an event. Then there's the statistical argument. Caltrans has some twenty million of these dots on California roads. If the dots were this kind of a danger, where are all of the dot-related crashes?
None of these are positive proof that the dots didn't cause some kind of hydroplaning-like event, but it makes me think this kind of event is really implausible. If the dots did cause the crash, I'd bet on the driver drifting into the dots, hearing the rumble, and jerking the wheel too hard. The Carrera GT's reportedly a touchy car to drive, and it has no stability control system. That's just speculation, though. Hopefully the investigation will turn up more and better information.
I'm of the opinion that like any crash, this one will take time to investigate and fully understand, and we should allow that to take place without speculating too much on what happened. And if road dots turn out to be the cause of this crash, well, then there's a first time for everything.
But the "road dot" theory just seems way off to me, enough that I would encourage everyone watching this case to swallow with a gallon of salt. We'll see what unfolds in the coming days and weeks.