Ramp signals (also known as ramp metering) are the traffic signals at motorway on-ramps that manage the rate at which vehicles move down the ramp and onto the motorway. With each green light, two cars (one from each lane) can drive down the ramp to merge easily, one at a time, with motorway traffic. Vehicles move forward every few seconds between green lights.
Separating the vehicles travelling down the on-ramp causes less disruption to the flow of motorway traffic, which in turn reduces the frequency of crashes associated with merging traffic.
Ramp signals help improve traffic flow, as well as capacity and safety on the motorway, while enabling more consistent speeds, safer merging and more predictable travel times. Ramp signals operate only when needed during busy periods. At all other times they remain switched off.
Managed priority lanes (transit lanes) for trucks, buses and car-pool vehicles (carrying two or more people per vehicle) also operate at some on-ramps.
What are the benefits?
The measured results show that with the ramp signals operating, there is an overall 15% improvement in travel speeds and a 5-15% increase in the rate of vehicles flowing on the motorway.
NZ Transport Agency has installed ramp signals at 61 motorway on-ramps on the southern, northern, north-western motorways and in the central motorway junction. Another 33 ramp signals are planned for the Western Ring Route between Manukau and Albany as it is being built.
The design of ramp signals takes queuing on the local road network into account when selecting the period of operation of the on-ramp meter signals and the rate at which traffic can be released onto the motorway at each on-ramp.