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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.
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.
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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.
How the illustration has been programmed
70 vehicles are randomly created between between 2m and 7m long with most being about 4m long.
Drivers are randomly created to have reaction times between 0.7 seconds and 1.8 seconds
with most being around 1.25 seconds. This is the thinking time plus the time to move between accelerator and brake.
Because the vehicles and drivers are created randomly, every time you visit or refresh this page you get a different set of vehicles and drivers. The overall behaviour of the traffic will be different each time.
The lead car can accelerate at 1.0g and decelerate at 1.0g
All other vehicles can accelerate at 0.2g and decelerate at 0.7g
Drivers do not start braking behind the vehicle in front until they've come within range. That is, they do not even contempate slowing down if they are more than 5 times their intended gap.
The maximum speed of the road is 89mph - no vehicles can travel faster than 89mph. This may give an unrealistic illustration at higher road speeds because following cars can't actually catch the lead car if it's doing 89 mph.
The traffic stream is considered steady when all cars are within 2mph of the road speed.
Please make contact to leave your comments.
References
The document referenced at www.visualexpert.com/Resources/reactiontime.html suggests a thinking time of 1.25 seconds for unexpected events such as a vehicle ahead braking.
Acceleration
0.2g is approximately 0-60mph in 13 seconds.
Deceleration / braking rate
The rate of deceleration used is 8.5ms-2 [0.85g]. This would represent a very high rate of deceleration
such as may be achieved by a car fitted with ABS when braking on a dry road.
A rate of deceleration of 4ms-2 [0.4g] would
represent the sort of deceleration that might be achieved on a wet road surface.
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