Perception and Response
Perception and Response Times: Daytime vs Nighttime
The U.S. National Highway Traffic Safety Administration states that nearly half of all traffic related fatalities occur in the dark,
despite only 25% of traffic travelling during darkness.
Braking: When you double the speed, the stopping distance increases by quadruple
Lighting: When you double the distance, the illumination reduces to one quarter
These two factors are working in opposition to one another with regard to driver safety for night driving. Driving at night with reduced
visibility and still travelling at the same daytime speeds means our capacity to perceive and respond to safety situations, even as simple
as observing accurately road surface conditions, is greatly reduced. Response times often more than double. Additionally many drivers
are not experienced in managing the increased driver fatigue experienced during night driving. Driver response times can be made
worse by poor visibility, fatigue, weather conditions, road surface, driving conditions and the vehicle condition.
At 100km/h, most drivers who see an object at 180m will have great difficulty to stop in time with standard headlights. However if that
same driver saw the object at 500m there would be plenty of time to stop safely with any vehicle type and in any conditions.
Please note that these are optimum stopping conditions not allowing for driver fatigue or poor weather conditions. 500m visibility
is a minimum requirement for any opportunity of safe collision avoidance with objects, hazards or even with other vehicles.
Improved vision reduces driver fatigue and increases the driver response capability to vehicle safety situations. Driving lights should
compliment vehicle headlight systems, providing greater distance and area of illumination that does not leave us blinded when we
have to dip back to low beam.
For this reason matching the colour and intensity of the driving lights to the headlight system is an often overlooked practice.
This often can be optimised by increasing the performance of the headlights with more modern light fittings.
Depending on the type and age of vehicle the headlight performance expected is:
low beam = 1 lux 60 to 100m, high beam = 1 lux 120 to 200m.
There are a number of contributing factors to the stopping distance of a vehicle, as listed and demonstrated below:
Human perception time
= 0.5 to 5 seconds, according to: light, visibility conditions plus distance and size of object/hazard
Human reaction time
= 1 to 2.5 seconds, according to: age, fatigue and attention levels
Vehicle reaction time
= 0.5 to 1.0 seconds, according to: system condition plus braking system efficiency to vehicle size
Vehicle braking capability
= Passenger car 40-60m from 100km/h
= Commercial vehicle 80-100m from 100km/h
= Heavy vehicle 120-150m from 100 km/h
According to: vehicle speed, vehicle weight, tyre condition and road surface