# Acceleration and deceleration: the facts

There was an article on BBC News today covering the court case of a man convicted of dangerous driving after driving his Renault Kangoo with an officer on the bonnet. Below is an excerpt.

"Christopher Evans, from Prestatyn, Denbighshire, reached speeds of 40mph (64kmh) with Pc Fran Donaghy holding on with one hand, Mold Crown Court heard.

"The officer had been trying to stop banned driver Evans, 40, in Hawarden in Flintshire when he drove off for 125m (410ft) before coming to a stop."

Let’s analyse the data.

America’s Drive and Stay Alive website claims that the stopping distance from 40mph is 80 feet (pure braking distance, not including thinking distance), or 24.4 metres.

The Renault Kangoo’s performance data suggests seven seconds will take it to 40mph. The graph therein suggests an average speed of around 24mph during this acceleration, which gives a total acceleration distance of 74.7 metres. Although I’m speculating, neither of these tests was done under the unusual circumstances of a police officer being attached to the car’s bonnet.

So 100 metres have been used up with full-on acceleration and deceleration, The following factors will likely increase this:

• The van had previously travelled over a grassed area, likely reducing its aptitude for acceleration and deceleration
• The van was likely not empty of stuff, again increasing its acceleration and stopping distances
• Unless he used his left foot for braking, there was likely a good few feet elapsed between the hard acceleration and the hard braking
• There was a police officer on the bonnet, for fuck’s sake. If this was good for aerodynamics, you’d see them on the front of F1 cars, right?

Given the above, I have to question how likely it is that he reached 40mph if his stopping distance was genuinely 125 metres.

I’m just saying.

2 Responses to “Acceleration and deceleration: the facts”

1. Nick Robinson on February 24th, 2009 08:35

Nice one!
I don’t believe that hanging on to the bonnet of a car is a good position from which to judge its speed anyway.

2. Dan on February 25th, 2009 09:37

Good point. Didn’t think of that. The police officer would be travelling at the same speed as the car.