By now we’re pretty well acquainted with ramp signals on motorway onramps across Auckland. You know the ones:
NZTA explains the reasoning behind ramp signals on this old page (which hasn’t changed since the NZTAs predecessor agency over 5 years ago):
Currently traffic on the motorway is disrupted by ‘bottlenecking’. This means whenever traffic enters the motorway and then shifts from lane to lane, it creates a slowing pattern as vehicles back up behind the on-ramp entry zone.
Most accidents on Auckland motorways happen during peak hours when traffic is stop-start, due to lapses in driver concentration and motorists travelling down crowded on-ramps vying for positions in traffic or trying to merge together.
Ramp signalling provides a smoother flow of traffic, minimising stop-start conditions by separating on-ramp traffic into streams of one or two vehicles. Ramp signals are designed to keep traffic flowing on the motorway and to reduce accidents.
Early analysis suggested that ramp signals improved the throughput and reduced the levels of congestion on the motorway:
Data gathered since the signals have been turned on has given the following results for individual sections of the Southern Motorway (SH1):
Curran Street northbound
- Improved northbound traffic flows on clip-on lanes on Auckland Harbour Bridge
- 18% – increase in throughput of vehicles
- 12% – improved peak period travel speeds
Wellington Street northbound and Northwest – North/Port-North
- 6% – increase in throughput of vehicles
- 4.5% – increase in travel speeds
Hobson Street to Market Road southbound
- 15% increase in throughput of vehicles
- 16% improved travel speeds
- Commuter traffic cleared 20-30 mins earlier during afternoon peak periods
- Safer merging and motorway incidents being cleared up to 15 minutes faster, to restore normal traffic flows
Between central city and Ellerslie-Panmure, Mt Wellington and East Tamaki Interchanges southbound
- Peak period traffic flows have been significantly improved
- Shortened periods of congestion
- Motorway is carrying significantly more traffic during peak periods than before
- Travel speeds have increased
The big question with ramp signals has always been whether it just “shifts the problem” onto the local roads. When ramp signals were initially introduced, Transit NZ was an agency solely focused on the operation of the motorway system and didn’t really care what happened on the local roads. As long as the congestion wasn’t on the motorways then it wasn’t their problem. NZTA has (fortunately) taken much more of a one network/system approach to transport in Auckland than this and work very closely with AT in the day to day management of the network.
I suppose the big question is whether the improvements to flow on the motorway network are sufficient to counter-balance the additional waiting on onramps and on the local roads that lead to the onramps. But more deeply, I think there are questions around whether we want idling cars (and their pollution) on local streets where everyone else is, or confined more to motorway corridors away from where people live, work, walk or cycle. Do we really want to privilege long-distance trips (those already on the motorway network or who access it at more distant onramps where the ramp signalling doesn’t happen or is on a faster cycle) over shorter distance trips from more central locations who get stuck on really long queues at the signals? These are all complex questions.
My gut feeling is that ramp signals are probably a good thing on balance providing they are actively managed to ensure:
- they aren’t in operation when they shouldn’t be.
- they respond to problems on the local road network.
- any benefit to the operation of the motorway network really does outweigh the impact they have on the local road network.
However I’m not sure if we are currently getting all of those benefits, in particular the impacts on local roads. Some of those impacts are bound to part of the reasoning for monstrosities like Lincoln Rd.