Ask Phoebe: Shedding some light on ramp signal policy

Photo / Martin Sykes

Can you tell us what the policy or approved practice is relating to when the motorway access lights are turned on? Access through the lights at Oteha Valley Rd seems to be controlled each morning whether the road between them and the next off ramp is clogged or empty but the lights at Highbrook Drive in the evening do not operate even when the road north crawls to Mt Wellington. I have crawled past non-operating lights at Curran St and there is a complete lane opening in front of the on-ramp at Tristram Ave. The red light at some points seems to be illuminated for less than a second but at others seems to vary between about four to about nine seconds. All the green lights seem to be illuminated for a ridiculously short period at all the sets of lights. Signs say only one vehicle is permitted to pass them per lane per green so a longer illumination period might encourage fewer drag race starts - saving the nation's fuel bill at no cost.

Please let us know who decides which lights should operate when and at what cycle time.

Derek Robert, Torbay.

Ramp signals are operated by the NZ Transport Agency from the NZTA/Auckland Transport Joint Transport Operations Centre at Takapuna. The system is designed to help keep motorway traffic flowing as freely as possible. Electronic sensors measure traffic volumes and the signals turn on when the system detects congestion. Signals may operate because of congestion around just one ramp, but they may also regulate the number of vehicles joining a relatively clear section of motorway because the sensors have detected congestion further along the network.

In contrast with street traffic signals, where the time of the green phase can vary, it is the time of the red phase that will vary on a ramp signal. It can be as short as two seconds and as long as 14 seconds depending on traffic volumes. The time for the green phase follows international best practice and is set at 1.3 seconds, enough time to allow one vehicle off down the ramp.

In regards to the locations which you mention: Oteha Valley ramp signals form part of the system that also includes the Greville Rd and Constellation Drive ramps. Ramp signals are activated in relation to data gathered from surrounding detectors. Highbrook Ramp signals are kept off in the evening peak because of the natural bottleneck constraint at Mt Wellington. Curran St ramp signals are now fully operational with the completion of the Victoria Park Tunnel.