Stopping Distance
This lesson covers:
- Defining thinking distance, braking distance and overall stopping distance for vehicles
- Factors that affect thinking distance and reaction time
- Factors influencing braking distance of vehicles
- Methods to reduce thinking and braking distances
- Calculating thinking distance based on reaction time and velocity
- Determining braking distance using equations of motion
Stopping distance
When a driver spots a hazard while driving, it takes time to process and physically hit the brakes.
There are three key distances to consider:
- Thinking distance - The distance traveled during the driver's reaction time before braking starts (m).
- Braking distance - The distance traveled after braking starts until the vehicle stops (m).
- Stopping distance - The total distance traveled from spotting the hazard to coming to a complete stop (m).
Stopping distance = Thinking distance + Braking distance
Factors influencing thinking distance and reaction time
The thinking distance is affected by factors which change the driver's reaction time.
The braking distance is affected by factors which influence the car's ability to slow down when the brakes are applied.
The key factors affecting stopping distance are:
| Factors affecting thinking distance | Factors affecting braking distance |
|---|---|
| Fatigue | Road conditions (wet, icy) |
| Alcohol | Condition of brakes |
| Distractions | Condition of tyres |
| Age of driver | Mass of vehicle |
| Speed | Speed |
Any factor which increases the driver's reaction time, increases the thinking distance.
Any factor which inhibits the car's ability to slow down, increases the braking distance.
Calculating thinking distance
The thinking distance covered is the reaction time multiplied by the velocity:
Thinking distance = reaction time x velocity
Where:
- Reaction time (s)
- Velocity (m s-1)
- Thinking distance (m)
Worked example: Calculating thinking distance
A vehicle travels at 20 m s-1. Calculate the thinking distance if the driver's reaction time is 2 seconds.
Step 1: Formula
Thinking distance = reaction time x velocity
Step 2: Substitution and correct evaluation
Thinking distance = 2 x 20 = 40 m
Determining braking distance
The braking distance depends on the initial velocity (u), the deceleration rate (a), and can be calculated using equations of motion:
braking distance = 2av2
Where:
- v = initial velocity (m s-1)
- a = deceleration (ms-2)
- Braking distance (m)
Worked example: Calculate braking distance
A car initially moves at 18 m s-1 and decelerates at 6 m s-2. Calculate its braking distance.
Step 1: Formula
braking distance = 2av2
Step 2: Substitution and correct evaluation
Braking distance = 2av2 = 2×6182 = 27 m