Over-driving your headlights is defined as – A scenario where the driver is moving at a rate of speed that their stopping distance is farther than they can see with their headlights. This creates a dangerous driving environment.
We have created two scenarios that illustrate the danger of “Over-driving Your Headlights”. To create the scenarios we need three numbers. A vehicles stopping distance, the illumination distances the headlights supply, and driver reaction time. The numbers used will vary depending on the driver and the vehicle.
SCENARIO ONE – LOW BEAMS
Driving with low beams a driver has 180 feet of vision to work with. At 40 MPH the vehicle is moving at the rate of 58.8 Feet/Sec. (40 MPH x 1.47), and needs 64 Feet to stop. The driver is confronted with an emergency at the edge of their 180 foot vision mark. If the driver’s reaction time is 1.5 seconds the driver would use up 88 feet (58.8 x 1.5) of the 180 feet of visibility getting their foot to the brake pedal, leaving the driver with 92 feet (180 – 88) to stop a vehicle that needs 64 feet to stop, leaving the driver with 28 feet (92 – 64) to spare. Sounds like a lot of room?
Consider that initially the driver was moving at the rate of 58.8 Ft/Sec it’s not much of a cushion.
In the same scenario except the driver’s reaction time is increased by half a second. The reaction time is now 2 seconds, which, at 40 MPH, translates into 117.6 feet to reach the brake pedal. (58.8 x 2) Which will leave the driver with about 62 feet (180 – 117.6) to stop a vehicle that needs 64 feet to stop, the driver will be 2 feet (62 – 64) into the emergency – not good.
It is a safe bet to say that the threshold speed on a poorly lit road with low beams, under the best conditions, is 40 MPH.
SCENARIO TWO – HIGH BEAMS
The same exercises with high beams – the driver has an average of 350 feet of illumination. Move the speed up to 60 MPH or 88.2 Feet/Sec. (60 MPH x 1.47), and needs 143 to stop. The driver is confronted with an emergency at the edge of their 350 foot vision mark. The drivers 1.5 seconds of reaction time will use up 132 feet to get to the brake pedal (88.2 X 1.5). The driver has 218 feet (350 – 132) to stop a vehicle that needs 143 feet to stop, leaving the driver with 75 feet (218-143) to spare. Sounds good– but consider that the initial speed is 88 Feet/Sec – again not much of a cushion.
Raise the driver reaction time a second, reaction time is now 2.5 seconds, which, at 60 MPH, translates into 221 feet to reach the brake pedal (88.2 x 2.5). Which will leave the driver with about 129 feet (350 – 221) to stop a vehicle that needs 143 feet to stop, which would leave the driver 14 feet (129 – 143) into the emergency – not good.
On a poorly lit road with everything in the driver’s favor 60 MPH is about the speed limit for driving with high beams, at 65 you’re tempting fate at 70 it can turn into a disaster.
A suggestion for those conducting EVOC programs
Measure the headlight distance- high and low beams, of not only the training vehicle but also a sample of the patrol vehicles. And if you haven’t done it already measure the braking distances of the training and a sample of the patrol vehicles. Work out the above scenarios with the numbers for your vehicles. It would be information you can pass on to your students.
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