Rapid car Deceleration

Most recent answer: 11/01/2016

I own a small, hatchback vehicle that weighs 3,200 lbs. I take part in Allstate's Drivewise program in which I plug a device into my vehicle, and it monitors speed, breaking, and mileage. Allstate regularly penalizes me for what they consider "hazardous breaking events" even when I am driving at extremely slow speeds. Allstate considers slowing down by 8MPH within 1 second to be dangerous. For example, they have penalized me multiple times for slowing down from a starting speed of 16MPH to an ending speed of 8MPH. I do not feel that slowing down by 8MPH within 1 second can be considered dangerous at these very low speeds. I believe that physics holds the answer to this question and I'm hoping that you can help me answer whether or not this type of breaking is actually dangerous. Thank you.
- Ashley (age 30)
New Paltz, NY, USA

I hate to say anything positive about an insurance company, but they may have some reasonable grounds. As you say, ordinarily decelerating from 16mph to 8mph in 1s creates no problems. It's not a great habit, however, for several reasons. That's an acceleration of about 0.4g, where g is the gravitational acceleration.

1. It's pretty abrupt, and may make it hard for someone behind you to react in time to avoid bumping you.

2. If the road is wet, it's actually not possible to slow down that quickly because the road-tire friction is too low. If you're in the habit of doing it, you may run into problems (i.e. the car in front of you) when the road is slicker than usual.

3. The insurance company cares about anything that indicates probable risk, even if it doesn't directly cause problems. Maybe people who suddenly slow down that much tend to be people who weren't keeping their eye on the road enough. 

At such low speeds, these problems are unlikely to lead to serious injuries, but they can lead to expensive collisions. If airbags trigger in a collision, that can cause injuries.

There's another reason too.

4. That's a fuel-inefficient way to drive. You've kept your foot on the gas longer than needed just in order to be able to slow down faster. 

So those are just some ideas. To find out whether the company is actually penalizing you an unreasonable amount would take someone more expert.

Mike W.

(published on 11/01/2016)