Driver Knowledge Tests

What causes traffic jams?

Phantom traffic and jamitons

One single car can start a traffic jam. Phantom traffic when cars on the road stop for no (apparent) reason. Traffic is a function of flow, density of the vehicles (i.e. how close they are together) and the speed of the vehicles.

When density reaches a certain point, one car braking can start a wave, called a jamiton, which travels backwards through the line of traffic at about 20 kph.

This video demonstrates it perfectly – 22 cars were put on a circular track and the drivers asked to drive at a constant 30 kph. It only takes one car to slow down to cause the cars behind it to clump together as the car behind slows its speed even more to compensate.

When density is high enough our natural reaction to the closeness of all those vehicles behind us is to slow down. This reduces the flow and speed, and increases the density, which can cause even more slowness. Eventually the whole thing grinds down to a low speed.

Density is increased by people joining a road from on-ramps, and this causes vehicles to slow down, i.e. people are generally bad at merging at speed. Then some people who have been held up in the left-hand lane move to the middle lane which causes the same problem in the middle lane – traffic backs up. Then some people dive from the middle lane into the overtaking lane and that slows down.

The optimal flow of traffic would be achieved if no one changed lanes unnecessarily and everyone maintained a constant speed that is perceived to be safe by drivers. This is why in some countries when the traffic is heavy, rather than speeding traffic up to move more traffic through, authorities use variable speed limits to slow it down slightly. The overall slowing of traffic reduces the incidences of panic braking, reduces the need for people to change lanes to try to get an advantage, and therefore increases the overall throughput or flow of traffic.

How can you help reduce a jam on a motorway?

If you see traffic backing up ahead of you, slow down early rather than suddenly, then try to maintain a constant speed without braking. By doing this you will help traffic behind you maintain a constant speed and you could help stop a jamiton or jamitino (successive jamitons) forming.

Walk, bike or take public transport if you can – one less vehicle on the road reduces the density and increases the flow

Try to drive outside of busy times so that you are not contributing to that critical mass of density when traffic suddenly becomes too much and grinds to a halt.

Traffic in cities

Traffic in cities is mostly controlled by traffic lights. Traffic lights try to optimise the flow for the majority of traffic, which means if you are travelling against or across traffic you might experience a lot of red lights.

To reduce traffic jams, respect intersections and don’t block them by queuing across them. Try to make your travel arrangements outside of rush hour – can you use flexi-time, for example. Take public transport if possible. Move closer to your work so you have to travel less distance.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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