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Traffic flow illustration. How a stream of vehicles responds to varying speed, traffic density and driver behaviour.

This interactive traffic illustration allows you to see how compression waves and stop start motoring occur in dense traffic. You can set the speed of the front car and watch the following vehicles adjust their speeds to suit. Also, by setting the average gap between vehicles you can simulate conditions where drivers are either forced to be dangerously close because of traffic density, or choose to be too close because condtions seem favourable. Watch the cars on the moving road to see what happens.

Lead Car (mph) Average Gap (secs) Drivers Challenge

Lead car can accelerate and decelerate more quickly than other cars.

The average gap (secs) between vehicles.                        

2 seconds should be safe.
2 seconds should be safe.

Safer Drivers: Add or remove a driver who leaves a gap of 3 times the average gap. (Yellow Cars)

Adjust the controls to achieve a flow of more than 100 cars per minute. See what happens if:
a) the lead car speeds up then slows down
b) you add or remove a safer driver
c) you add or remove an unsafe driver

Less Safe Drivers: Add or remove a driver who leaves a gap of half the average gap. (White Cars)
road speed - mph/kph: 
flow  - vehicles/min: 

 





Factors in this traffic illustration

Tracking speed
To keep the action on screen the camera moves to keep pace with the lead car; effectively this is the road speed – slower cars dropping behind and faster cars catching up.

Traffic Flow
Traffic flow is measured here in vehicles per minute. You can see that the flow is affected more by the gap between vehicles than by changes in speed. Unfortunately, to achieve high flow rates the gap between vehicles must become dangerously reduced. An average gap of 0.7 seconds puts the stream at a critical point because this is less than the thinking time needed before applying the brakes.
Traffic density
The slider adjusting the average gap between vehicles should be used to set the overall traffic density. Sometimes the traffic is dense because of the total number of vehicles entering the lane. Sometimes the traffic is dense because of temporary events, such as passing slower traffic. Sometimes the traffic is dense because good driving conditions lull drivers into a false sense of security. The overall flow rate is determined by the speed and density of the traffic.

This illustration cannot accurately represent all the factors that affect a stream of traffic - use it only as an aid to understanding your own experiences.

Drivers Drivers vary a lot. The model starts with the assumption of a normal distribution where drivers on average expect to leave a gap of 2 seconds. To vary this model you can intersperse drivers who leave three times the normal gap or half the normal gap. Altered drivers become coloured to make them more easily identifiable.

Journey time versus flow rate When the road is not busy and a driver is free to travel unhindered, the journey time is dependent only on speed. In busy conditions journey time relies on the flow rate and drivers have no option except to help maintain the flow. To improve the flow rate drivers can reduce the gap between themselves and the vehicle ahead but this could then cause a crash and reduce the flow rate to zero. Sometimes drivers are forced to be closer than is sensible and most will take gradual action to correct this situation.