Across the road network there are some locations which we have received high numbers of feedback and comments for. These usually ask why are ramp signals turned on when the motorway appears to be free flowing or that getting onto the motorway is very slow and the onramp and arterial road is congested.
The signals are on from around 7am to 7pm, Monday to Friday, with some variation depending on when congestion starts or finishes. Queues only form during the morning and afternoon peak periods due to demand overloading the capacity of the on-ramp. The motorway section between Gillies Ave and Khyber Pass is known to have a high accident rate due to the lane configurations and on and off-ramps. The introduction of ramp signal here has reduced the motorway crash rate by 26%.
One benefit of operating these ramp signals during the days is that fewer vehicles merge at any one time. This allows vehicles typically travelling at a higher speed that are wanting to exit the motorway at Gillies Ave to move across more easily.
Congestion has been observed at Green lane Interchange in the southbound direction around 7:45am to 9:15am. The ramp signal at the Grafton Rd on-ramp has limited vehicle storage space, which unfortunately is not as effective as it could be when managing peak congestion. The other on-ramps through the Central Motorway Junction (Hobson Street, Symonds Street, Grafton Road, and Khyber Pass) are also activated so that traffic congestion further down the motorway can be relieved.
The operation of this ramp signal during the morning peak gives the appearance of operating for no good reason. It is in fact trying to help the congestion as far away as Green lane by slowing the traffic arrival rate into the congestion area.
St Marks Road and Green lane Rd ramp signals are grouped together, to manage and share the load for the bottleneck congestion area just before the merge point at Green lane. As a critical location on the network, congestion and queues form very quickly and often traffic slows to a very low speed. The low speed reduces the amount of vehicles passing through the congestion point and in turn creates more congestion. This then increases overall travel time for all motorway users. The St Marks on ramp helps manage this flow bottleneck but is given a slightly lower priority than the Green lane ramp due to lower onramp demand flows and the complex nature of the Green lane interchange.
After the construction of the new northbound on-ramp from Karaka, changes were made to the signal timings due to the merging points from both city bound on-ramps being very close together causing severe flow disruption and congestion during the morning peak. Approximately 40% of the total number of vehicles entering the northbound motorway now comes from the Karaka on-ramp. This is a reflection of the growing number of new residential properties being developed west of the Papakura interchange.
The changed road layout now has more lanes for a short distance than was previously available. This means more vehicles can enter the motorway at any given time as there are now two motorway entry locations. However the downstream motorway lanes have not yet changed from the two existing lanes so that the total capacity of this section is still the same. The only real way to manage at present the balance between the two closely spaced on-ramps and the motorway flow is to use long red times at the ramp signals. The higher red time controls the number of vehicles entering the motorway. Road users can expect delays until the extra motorway lanes are constructed through the Southern Corridor Improvement project(external link).
