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Stopping Sight and Driver Reaction Time

Stopping Sight Distance

S=dr+db

S=stopping sight distance
dr=driver reaction distance (decision distance - combined observation, identification and decision distance)
db=braking distance (action distance)

dr=v.tr

db=(v*v)/(2g(f+-G))

v=speed
tr=driver reaction time (varies from 2,5 to 2,8sec)
g=acceleration of gravity
f=coefficient of friction between tyres and pavement (varies with speed – for wet, glazed asphalt it varies from about 0,45 at a speed of 10km/h to 0,2 at a speed of about 70km/h)
G=average grade

The above distances depend on whether a vehicle goes into a skid or not – distances increase drastically when vehicles go into a skid.


 

 

The type of vehicle (vehicle mass – together with speed relating to momentum) also plays a role – for example : in emergency situations a light passenger car doing in the order of 100km/h needs about 60m to stop, a truck weighing less than 5t would require about 70m and a truck weighing between 5t and 10t would require in the order of 80-90m (both doing about 100km/h). (In all these cases vehicles do not go into a skid situation and distances given do not include driver observation and decision distances).

Speed plays a major role in driver observation, recognition, decision and reaction time. In the case of a hazard or an incident on the road, the driver of a medium size motorcar driving at a speed of 120 km/h will need a total distance of about 227 metres from the point of observing a hazard and taking evasive action until the vehicle comes to a standstill. For a driver driving at a speed of 180 km/h this distance more than doubles to about 480 metres. Information on the total observation, recognition, decision and reaction distances required for various sized motorcars and speeds on a flat section of road are given in the graph below.

 

 

Required Stopping Distance - metres
Decision distance plus Action distance
Speed
km/h
Small
Car
Medium
Car
Large
Car
Heavy
Vehicle
60 60 66 77 116
70 78 86 101 154
80 99 109 129 197
90 121 135 159 246
100 146 163 193 300
110 174 193 230 359
120 203 227 271 424
130 235 263 314 495
140 269 301 361 570

 

 

Required Stopping Time - metres
Decision Time plus Action Time
Speed
km/h
Small
Car
Medium
Car
Large
Car
Heavy
Vehicle
60 5.73 6.44 7.76 12.37
70 6.50 7.32 8.87 14.24
80 7.27 8.21 9.97 16.11
90 8.03 9.09 11.07 17.98
100 8.80 9.97 12.18 19.86
110 9.56 10.86 13.28 21.73
120 10.33 11.74 14.38 23.60
130 11.09 12.62 15.49 25.47
140 11.86 13.51 16.59 27.34

 

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Brakes/Braking and Road Safety

 

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