Traffic planners can dramatically cut journey times by temporarily reducing the number of cars that enter a motorway. Called ramp metering, the system asks for less haste, but offers more speed. A team of European researchers have developed and tried out a new system to make motorway traffic flow more smoothly during rush hours.

It is the essence of rush hour. You are cruising along the motorway at 120 km per hour, eating up the tarmac. Suddenly, as you approach a motorway on-ramp, incoming cars start merging with the main traffic flow.

Drivers across all four lanes start touching the brakes, just slightly, forcing drivers behind them to do the same. Rapidly, the number of cars slowing down reaches 10, 20, 50, 100, more. It is a congestion shockwave, zooming upstream.

Within seconds, a traffic jam forms and hundreds of cars are rapidly adding to the queue. Worse, the exits are blocked, compounding the problem. Thousands of cars are stopped; just so a few dozen can enter the road.

But there is a simple solution. Special traffic lights, called ramp meters, can smooth on-ramp flow, keeping the mainstream flowing freely. It can reduce – and sometimes even eliminate – motorway congestion caused by uncontrolled access.

Even better, it means shorter journey times, even for the drivers held up in the on-ramp, thanks to alleviating congestion.

“We have seen 10 to 20% improvement in journey times in real-world tests,” says Professor Markos Papageorgiou, director of the Dynamic Systems and Simulation Laboratory (DSSL) at the Technical University of Crete and a researcher with the EURAMP project. “But from our simulations it’s possible, in optimal situations, to achieve journey times 30% to 40% faster.”

That represents a staggering potential economy. Take the Amsterdam ring road, where traffic reaches 2,200 vehicles per hour per lane, all getting to their destination dramatically faster. Petrol, carbon-emissions, motorway construction, wasted time … there are savings everywhere. It represents millions of euros for Amsterdam alone.

That is the why of ramp metering.

The trick is to know where and when it will be effective, how it can be deployed, and what tools are required. Enter EURAMP, an EU-funded research initiative set up in January 2001.

EURAMP analysed the state of the art, developed better tools for the automatic control of ramp lights, tested their effectiveness in real-world conditions and developed guidelines for ramp meter implementation.

The intelligence of the system
“There are local ramp metering applications; we developed a popular one called ALINEA, and they control individual meters,” explains Papageorgiou. “But we also developed a system that coordinates the operation of meters along a motorway. This is a more intelligent system. Ramp metering may be very effective.”

Just how effective was demonstrated in a test in Israel. It was a first for the country and it required new regulations to legalise on-ramp metering. “They started from scratch, and metered just one ramp, but that alone improved drive times by 10%,” notes Papageorgiou. A Paris test revealed improvements of up to 15% in journey times.

“The storage capacity of on-ramps is a key constraint,” emphasises Papageorgiou. “You need to be able to hold several hundred cars on nine or 10 ramps to get maximum return for the effort.” Once the ramp fills up, the system must cut out, or on-ramp queues will feed into city streets. It is a constraint, but it is not an expensive one to overcome. New ramps can be designed for greater capacity, existing ones can be widened.

But the greatest potential gain for the lowest costs is probably in inter-motorway ramp metering.

“Right now metering on a motorway is banned in most countries. But by metering on-ramps within a motorway system, ramps that take drivers from one motorway to another, we could reduce congestion for little cost,” remarks Papageorgiou. “They are generally quite long, so they offer a lot of storage. And they are already there.”

Ramp metering is not a cure-all. It will not work in every circumstance. “If congestion is caused by spillback from an exit ramp, metering on-ramps will not help,” warns Papageorgiou. But uncontrolled on-ramps remain the major cause of congestion.

Now a number of cities have enthusiastically adopted the EURAMP system. Paris is extending it and Israel will develop more metered ramps. Even groups not associated with the project are taking the system on board, notably in Australia and the UK.

All of them are working hard to take the haste out of rush hour.