Driving tests

How do traffic jams form?

Traffic jams happen when the density of traffic overcomes one single driver’s comfort in travelling at that speed in that close a proximity to other vehicles. The driver brakes or slows down which causes the vehicle behind to brake, and then the vehicle behind that, and so on in a wave called a ‘jamiton’ which travels backward at around 20kph.

You can see it in this video whereby 22 cars were driven on a circular track. Drivers were instructed to drive at a constant speed of 30kph. However, if one car slows down, the others behind it have to, too, and this causes clumps of traffic to form.

So, it’s possible for one single car to start an entire traffic jam. Traffic density doesn’t even have to be that high: a vehicle trying to merge from a motorway on-ramp at a slower speed than the traffic on the motorway will have the same effect. The reason why we have on ramps is to allow vehicles to get up to speed before they get to the motorway.

The cause of a traffic jam is a function of speed and density. As density increases, speed tends to decrease which causes more density.

What causes traffic jams?

  • Merging at a different speed to traffic already on a motorway
  • Not being prompt away from traffic lights (this causes fewer cars to get through than would normally be able to get through, delaying journeys)
  • Unnecessary lane changing (when you change lanes in heavy traffic you cause traffic behind to brake)
  • Driving too fast (you cause other drivers to react by slowing down)
  • Driving too slow (you unduly hold up other drivers and cause them to change lanes to get past you).
  • Queuing across intersections (this blocks other traffic from moving)
  • Running red lights (traffic light phasing is tuned to allow the maximum traffic through, and if you run a red light you cause other drivers to be cautious away from the line on the green, which affects the amount of traffic that can get through on that phase).
  • Leaving several car lengths between you and the vehicle in front when you are waiting at the lights (it means less traffic can fit on that stretch of road, and that there will be much more gap between you and the vehicle in front when you move away on the green light)
  • Rubbernecking – looking at a previous accident

The best traffic flow is achieved when all vehicles travel at the same speed and avoid changing lanes unless absolutely necessary. Some countries use variable speed limits which are displayed on overhead gantries which bring the speed limit down slightly when traffic density increases. While this sounds counter-intuitive because less traffic can flow through in the same period of time, it actually improves traffic flow to a point because drivers feel more comfortable in the dense traffic at a slightly slower speed.

In a city environment traffic jams are caused by too much traffic not being able to get through traffic lights. Ultimately there are situations on urban roads without traffic lights where the volume of traffic causes drivers to slow down, particularly where two lanes merge into one, or where there is a side street that traffic has to force its way out of, or a roundabout which naturally slows down the flow of traffic.

How to avoid traffic jams

The best way is to avoid driving, however, if you have to drive, try these tips:

  • Use flexi-time: if you can start work later or earlier, and leave when it’s not rush hour, try that.
  • Take a bus the last few kilometres: if you have to drive because public transport is rubbish where you live, try using park-and-ride facilities where the bus takes you the last few kilometres
  • Take your bike the last few kilometres: park somewhere safe then take your bike into the congested areas
  • Use traffic camera websites or apps that show you traffic flow so you can avoid congested areas
  • Change meeting times if you see traffic is backed up
  • Work from home if possible
  • Walk if a meeting is nearby
  • Move closer to where you work, or to a place where you go against traffic when going to work
  • Put the kids on a bus rather than driving them to school, or drop them off at walking school buses
  • If you find yourself in a traffic jam, try to even out the braking/accelerating cycles so that you help prevent another jamiton forming behind you. If you can keep a constant speed, you help clear the traffic jam.
driver training

Darren is a member of the Institute of Advanced Motorists and the NZ Motoring Writers' Guild

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