Speed and Safety

Some numbers to remember:

Stopping a car has two stages: 

1.  driver sees a reason to stop and decides to press the brake pedal; 
2. Driver applies the brakes, the car decelerates until it stops

During both stages car is still moving. Distance travelled while driver decides is called thinking distance. And while the car decelerates is called braking distance. The addition of these two is stopping distance. 

Stopping distance = thinking distance + braking distance.

Thinking distance is affected by:

• Lack of attention and concentration;
• Influence of alcohol or drugs or some medications;
• Tiredness
• Speed 

Braking distance is affected by:

• Road condition: like if it is wet or icy
• Tire condition
• Condition of brake disk and pad
• Speed 

High speed affects both thinking distance, and braking distance. Also the large deceleration required to stop a car moving at high speed, can use the brakes to overheat, or cause the driver to lose control of the vehicle.

Measuring human reaction time:

Person A holds a ruler at one end. 

Person B keep their hand at the other end of the ruler with a gap between their index finger and thumb.

Person A releases the ruler without telling person B.

Person B tries to catch the ruler. 

We record the number of centimetres passes from person B’s hand. This will convert to reaction time.