Physics

Braking Distance and Reaction Time



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A distracted driver will have a greater reaction time than a non-distracted driver. A distraction to a driver will increase the drivers' reaction time and reduces the ability to respond to an emergency situation. The driver takes longer to react and more time passes between seeing the hazard and starting braking, so the car travels a greater distance before it comes to a stop.
Two important factors to take into account for calculating stopping distances are reaction time and breaking distance.

Reaction time
For average drivers it takes 1.5 seconds to react to an emergency situation. For a distracted driver it may take as long as 3 seconds. A focused driver driving at 60km/hr will travel approximately 25m before they react, and a distracted driver driving at 60km/hr will travel approximately 33m before they react.

Braking distance
The breaking distance of a car depends on a number of variables. The slope of the roadway; a car will stop more quickly if it is traveling uphill because gravity will help slow the vehicle. The frictional resistance between the road and the tyres of the car is also important. A car with new tyres on a dry road will be less likely to skid and will stop more quickly than one with worn tyres on a wet road. If the slope and frictional resistance are equal, the factor that has most influence on braking distance is the initial speed.

Formula used for calculating braking distance:

d = V /(2g(f + G))

Where:
d is the Braking Distance (m)
g is the Acceleration due to gravity (9.8m/s^2)
G is the Roadway grade
V is the Initial vehicle speed (m/s)
f is the Coefficient of friction between the tires and the roadway (u)

A more simple formula used to calculate braking distance can be derived from a general equation of physics. Ignoring friction, and the roadway grade

v = u - 2ad

where:
v is the final velocity (m/s)
u is the initial velocity (m/s)
a is the acceleration (m/s^2)
d is the distance traveled during deceleration(m)

Since we know that v will be zero when the car has stopped, the equation can be re-written as

d = u/2a

The total distance it takes for the car to come to a stop can be found by adding the reaction distance to the braking distance.
For more information on the Coefficient of Friction go to:
http://www.helium.com/tm/210140/coefficient-friction

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