When traffic lights are installed on motorway on-ramps this is for ramp metering or ramp signalling. It’s used to control the rate at which traffic can enter a motorway by holding traffic back at a set of lights on the on-ramp, then releasing them gradually. New Zealand has had ramp signalling since 2004. The main objectives are:
- Control the number of vehicles that are allowed to enter the motorway in a given timeframe
- Reduce the demand for motorway travel
- Break up platoons of vehicles released from an upstream traffic signal (i.e. a bunch of vehicles that all get released at the same time at one ramp further back up the motorway which makes it more difficult to merge)
- Make it easier to merge (released vehicles have a clear on-ramp in which to accelerate and get up-to-speed, and they will enter a motorway which has a more measured flow of vehicles rather than large platoons).
What are the benefits of properly implemented ramp metering?
Highway authorities assert that ramp metering can:
- Increase motorway throughput and productivity (i.e. get more vehicles through in a shorter space of time)
- Increase the overall speed of the motorway
- Increase the safety of merging
- Decrease fuel consumption and vehicle emissions, especially if vehicles have automatic start/stop technology, or are hybrids which turn petrol/diesel engines off when stationary
- Priority can be given to vehicles with multiple occupants by having a transit (T2 or T3) bypass lane.
Ramp metering gives an advantage to drivers who drive a longer distance on the motorway and enter the motorway at ramps with light traffic. They get the benefits of faster overall flow of traffic on the motorway. Ramp metering disadvantages drivers entering the motorway at popular and congested ramps as it delays their entrance onto the motorway.
Overall, ramp metering is designed to extend the life of the existing motorway network by delaying gridlock. Research in Auckland showed a 25% improvement in congestion duration and traffic speed, an 8% increase in throughput and a 22% reduction in crashes. The annual benefit for each installed ramp metering system is a reduction of around 26,000 hours of delay and six crashes, resulting in around a NZ$2.2m saving per installation. Auckland uses the SCATS system.
What are the disadvantages to ramp metering?
- Holding traffic on the ramp reduces the distance available to get up-to-speed with the traffic already on the motorway. If drivers cannot match their speed with traffic as they merge it can exacerbate congestion as it causes drivers to brake and that braking ‘wave’, called a jamiton, travels backwards through the traffic at around 20kph. This article about how traffic jams form explains it. If ramp signalling is done properly, then the signals will only activate when the average traffic speed on the motorway drops to approximately that which the joining traffic can reach
- Ramp metering means that new on-ramps must be built longer which is more expensive in road construction.
- The traffic lights at on-ramps require maintenance and power.
- Long vehicles find it difficult to get through on one phase.
- If a motorcyclists filters though the middle of two lanes to the front of the queue there is confusion over whether the other two vehicles that were at the front of the queue are allowed to go.
- Sometimes ramp metering appears to be in operation for no reason, leading to poor public perception, and possibly more emissions and unnecessary delays.
What types of ramp metering are there?
Ramp metering can operate in two timeframes:
- Peak time: lights are activated either automatically in response to a certain traffic level on the motorway, or manually by a traffic control centre. They warn of their impending start by flashing orange for a few seconds. When there is no need for ramp metering all of the lights go out.
- Continuous: lights are activated the whole time.
In New Zealand, all ramp meters are peak time although the hours of operation for each set of lights varies.
What types of ramp metering algorithms are there?
- Pre-timed: metering rates are fixed (e.g. one phase very 5.5 seconds). They are determined using historic data.
- Traffic responsive: metering rates are variable depending on how much traffic there is right now. It uses sensors on the motorway and arterial roads to determine this.
- Restrictive: restrictive metering rates are below the non-metered level, ensuring that less traffic can enter the motorway than without metering to try to ease traffic on the motorway
- Non-restrictive: metering rates are set to the average non-metered demand level for that specific ramp
- Less restrictive queue override: If there’s a queue on the on-ramp, maximum metering rate is implemented as there’s little to no benefit in holding vehicles up. Freeing up the traffic to join the queue increases the capacity of the on-ramp (i.e. more vehicles can wait on the on-ramp, removing them from the arterial roads)
- More restrictive queue override: If there’s a queue on the on-ramp the system slows the metering down or shuts it off, until the queue has dissipated.
- Integration between the ramp and upstream traffic signal: traffic lights at the approaches of the on-ramp on arterial roads are also managed to control the flow of traffic onto the on-ramp.
The SCATS program uses integration between ramp signals and upstream traffic signals.
What types of ramps are there?
Within the on-ramp there may be room for one or two vehicles (side-by-side) and one vehicle per green phase is allowed to pass.
Some on-ramps, e.g. Mt Wellington in Auckland, also feature a transit lane.
What types of signals are there?
Signals are the usual traffic light phase: red for stop, green for go and amber to stop. There could be multiple sets of lights at eye level for traffic at the head of the queue and on a gantry for approaching traffic. The image below shows the on-ramp at Mt Wellington, Auckland, heading north. The transit sign can be seen on the left of the gantry for the bus/truck/motorbike/transit lane (T2). Two lanes of other vehicles are held at the gantry waiting for the green light.
There could be a sign on the feeder road showing that ramp metering is on.
This video shows a simulation of SCATS. Watch it in double speed as it drags a bit in places: