From the Book

“Something’s not right,” said Will, flicking back to the page with the skid calculations. “According to this, Jeff was driving at 80 km/hr, braked hard for thirty metres and still hit the wall at 77 km/hr.” He ran his eyes over the page and stabbed at it with his right index finger.

“What?” said Drake and Besra.

“They’ve done a NASA,” said Will. “Mixed up the units. Whoever did this used kilometres and metres in the same calculation.”

The Problem

Will and Besra are at the police station going through the investigation report for the crash in which Will’s uncle Jeff died. They’re examining the calculations for the car speed and its impact velocity with the tunnel wall.

Determining the speed of Jeff’s car

The crash report has surveillance camera footage of Jeff’s car moments before the crash. The two camera frames are from one second apart, and by measuring the road the police have estimated the car has travelled about 22 metres over that time period.

Car speed = distance travelled / time taken

Car speed = 22 metres / 1 second

Car speed = 22 m/s

To get the speed in km/hr, need to convert from metres to kilometres by dividing by 1000, and from seconds to hours by multiplying by 3600:

Car speed = 22 m/s x 0.001 m / km x 3600 s / hr

Car speed = 79.2 km/hr


Determining the impact speed

To calculate the impact speed of the car with the wall, the report uses the formula:


  • v_initial is the initial speed of the car
  • g is gravity
  • d_braking is the estimated braking distance, which can be estimated from the length of the skid marks – about 30 metres.
  • f is the friction coefficient. The report has used a coefficient of 0.7.
  • G is the road gradient (the slope of the road) – in this case 0.1, since the road is slightly uphill

The report plugs the numbers into the formula like this:


Will realizes something’s up – braking for 30 metres and only reducing the speed by less than 3 km/hr doesn’t make a lot of sense. After a bit of thinking, they realize that the report has mixed up different units in the formula:

  • The initial speed of the car has been entered in units of km/hr
  • The values for gravity and the skid mark length have been entered using units of metres (m/s/s for gravity, just m for the skid mark length)

Besra re-calculates using consistent units – metres:


With consistent units, they calculate that the car had slowed down a lot by the time it hit the wall. This new information adds to their suspicion about the report.

Speed Cameras on Downhill Roads


A lot of people complain when police put speed cameras on downhill roads. Let’s see what happens if we add a downhill gradient of 0.1 to the calculation above:


When the slope is downhill, it takes longer to slow down – after a 30 metre skid the car has only slowed to 41 km/hr (instead of the 13 km/hr in the uphill case).