Traffic flow. How vehicles respond to varying speed, traffic density and conditions. The phantom traffic jam.
Vehicles Per Minute
How phantom traffic jams occur
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 4.0m
and 7.0m long with most being 4.5m long. This is to represent traffic in a lane that does not expect lorries or very long vehicles, normally the outer lanes.
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.
Education, carrot and stick. There are plenty of ways of educating drivers using articles in newspapers, on television and other media. The carrot would be that the inside lane will always be best maintained and give a more comfortble ride. (Obviously some costs here but considering the alternatives of building more lanes or converting the hard shoulder, the cost of keeping inside lane well maintained AND making sure that people know, must surely be cheaper.) The stick is for people who go too slow or too fast or simply don't move over to let others overtake. This also applies in average speed check zones where speed differences are usually small but frustrating.
And maybe (tongue in cheek)
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
Traffic flow is measured here in vehicles per minute. You can adjust the circumstantial traffic flow using the buttons. This way you can simulate the effect sudden changes caused by vehicles joining or leaving the flow from junctions or extra cars in your lane as they need to move over.
The safe gap
If all vehicles could maintain a 2 second gap the flow would be just
under 30 vehicles per minute, depending on the general speed. The vehicles you find in your stream
will have a random mix of desired gaps, ranging from 0.2 (crazy) to
4 (very safe) seconds. By adjusting the flow, say from 30 VPM to 40
VPM, you would force all drivers to compromise their desired gap to
accommodate the new circumstance. This would cause the "comfort
factor" to be changed.. The "comfort factor"
is a notional measure intended to indicate the variation from the
driver's preferred gap. Your desired safe gap may be compromised by
events beyond your control.
The ability of cars to brake in response to an emergency is determined by the grip available. In dry conditions we'll assume this is 100%. Grip reduces through damp and wet conditions to about 10% in icy conditions.
The illustration shows a single lane of traffic. If the conditions remained steady this lane could be be quite safe. HOWEVER, the reality is that the conditions are not steady. Even in a 50 mph average speed check zone you'll occasionally find some fool doing 40. This has the effect of forcing others into outer lanes and thereby severely affecting the density in those lanes and necessitating emergency slowing to accommodate the traffic sqeezing in. Count that as 2 effects: denser traffic and car in front slowing to recover a desired gap. You can play with the buttons to see what happens. And, of course, it's the same when a lorry occupies the middle lane on a busy motorway - sometimes for 10 minutes - in a selfish attempt to overtake another lorry going slightly slower, or even sadder, overtaking a car! People just don't realise that driving slowly is not safer: it is more likely to cause an accident than if they just drove at the speed of the majority.
In the end it's not about affording a few people a fast journey but more about affording a lot of people a safe journey. When motorways are nearly empty it's quite possible for a speedy driver to thread through the gaps and achieve a quick journey time (often at the expense of others), but once those gaps are gone it's a question of getting vehicles per minute along the road - and that can only be achieved with a smooth traffic flow and no one needing other people to slow down to make room for them. Of course, this is easier said than achieved, given the variable densities that occur at rush hours and at junctions.
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