Ramp signals – those traffic lights on motorway on-ramps that are designed to limit the number of vehicles that enter the motorway and thereby help keep traffic flowing, have long been a controversial transport initiative. They were first introduced on Auckland’s motorway system between 2006 and 2009, and are now present on nearly all busy on-ramps.

NZTA’s explanation of ramp signals focuses entirely on how they help limit congestion on the motorway network:

Ramp signals are traffic lights, which have been installed at the end of motorway on-ramps to manage traffic flow onto the motorways during peak periods and other busy times. These signals operate automatically when they are needed to improve traffic conditions on the motorway. Ramp signals are widely used internationally and have been shown to improve motorway traffic conditions, resulting in more consistent travel times, safer merging and fewer accidents. Electronic sensors built into the road detect when traffic conditions change, allowing the system to automatically activate and adjust each ramp signals timing sequence in response to current motorway traffic conditions.

And it does seem as though ramp signals have been fairly successful at this job:

Auckland motorway data gathered since the introduction of ramp signals has shown the following results: 12% improved travel speeds across the motorway.

18% increase in throughput onto the motorway.

Reduced merging incidents: around 32-34% outbound and 17% inbound

Faster response times to incidents by Emergency Services due to improved traffic conditions.

Motorway incidents being cleared up to 15 minutes faster. Large volumes of vehicles entering a section of motorway will cause traffic to slow and create a ripple effect on the motorway. Ramp signalling provides a smoother flow for traffic entering the motorway, by breaking up these large volumes into more manageable numbers. Accidents that occur on Auckland’s motorway, happen during peak hours when traffic is stop-start which adds to the congestion and reduces overall efficiency of the network increasing journey times.

Of course nothing comes or free and this “improvement” to motorway travel speeds and throughput comes at a cost, which is much longer queuing time to get onto the motorway in the first place. In many places the queue is simply shifted from the motorway to the on-ramp and the local roads feeding into the on-ramp. When the signals were introduced this ‘shifting the problem’ seemed fine for NZTA, because the queues on local roads were Auckland Transport’s problem.

So if all we cared about was to keep the motorways flowing, then ramp signals are a reasonably good way to help with achieving that goal. Of course this is absolutely not all that we want to achieve from the transport system, and it seems to me that ramp signals have quite a few perverse outcomes that call into question whether they are fundamentally flawed. Let’s run through a few big issues:

Ramp signals prioritise longer trips over shorter trips, thereby incentivising more vehicle travel. Ramp signals push traffic queues off motorways and onto local roads, where the queuing cars disrupt public transport, walking, cycling and where the pollution from these queuing cars is much closer to people and therefore more likely to harm their health. Ramp signals can discourage use of the motorway network, resulting in more traffic on local roads to avoid the congested onramps.

It’s this last issue I’m going to look at in more detail – with a particular focus on how ramp signals in spaghetti junction create a stupid incentive for people to drive right through the city centre.

The ramp signal in question is where the SH16 ramps from the Northwest Motorway and the Port join with the Northern Motorway. This site is particularly notable in that the motorway picks up a lane here so it’s not like traffic is even merging with the traffic from further south on SH1. You can see in the afternoon peak how that leads to massive queues:

The implication of all this is that drivers get put off using the motorway for this cross-town trips and instead drive through the city centre or through other roads such as Ponsonby Rd.

Making a change where this ramp signal was removed would, of course, mean that other parts of the motorway may end up a bit more congested – especially trips travelling from south of Grafton Gully through to the Harbour Bridge and beyond. But this comes back to the fundamental question of what’s best for Auckland as a whole. Given the council’s goals in making the city centre more people friendly, getting as much traffic as possible out of the city centre is a key part of that and will very good for Auckland. As for those long distance trips, let’s not forget we’ve just spent billions building an alternative state highway route through the region.

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