by Glen Smith
Who said we shouldn’t be improving our roads? Road improvements increase road network capacity and so overall transport capacity. Plus many trips can most efficiently, or only, be made by car. But the science clearly shows that, as a city grows, if you ONLY (or overwhelmingly) build roads you inevitably end up with crippling congestion (Los Angeles is an example) that defeats what you are trying to achieve.
This is because mass car transportation is ultimately self-destructive unless heavily supported by alternative, more efficient modes. Cars are very comfortable and convenient and it would be fantastic if every trip could made efficiently by car. But traffic flow dynamics means this is impossible. A growing city has a limited capacity to accommodate inefficient mass car transportation.
The science behind this is relatively straight forward. A road corridor has a fixed maximum capacity (the ‘critical density’) and when corridors are combined into a network this in turn has a corresponding fixed maximum capacity (the ‘network critical density’). This forms an inescapable ‘ceiling’ on the number of car trips a city/ region can accommodate.
If you try to exceed this, the network rapidly disintegrates into congestion. For some of the basic science, see my 2021 article “Being Grateful for Cycleways”.
So what do we do when trips reach/ exceed this limit?
You can try to increase road network capacity by building more and more roads (the 1950s model) but experience shows this leads you down a very problematic path. Because trips occur across the network (not just arterials) you have to do this everywhere, taking up ever more city space and promoting a sprawling urban design. This sprawling design in turn necessitates ever increasing car trips to get around, which fills up most of the extra capacity you have created, so you end up in a spiral of sprawl and car dependence, with an urban landscape dominated by the motor vehicle. Plus research shows the additional capacity costs an ever-escalating amount (like 3 billion dollars) for an ever-decreasing increase in network critical density until finally no extra increase in capacity is possible. Most cities have abandoned this approach.
The alternative is to recognize car transportation as a limited resource and put in place strategies to reserve this capacity for trips that are of high value, and which can’t be easily be undertaken by alternative, more efficient, modes. This commonly involves congestion charging and investing in high-quality public transport.
Instead in NZ we heavily subsidise car trips (effectively paying people to drive) encouraging people to take low-value trips, and we fail to plan the high-quality PT corridors that will attract trips and keep the parallel road network functional.
The tendency for car networks to self destruct is so powerful that these high-quality parallel PT corridors should be seen as an essential part of efficient road development and incorporated into planning.
The June 2013 Ngauranga to Petone rail washout gave us a sober reminder of what a road corridor deprived of its supporting parallel PT corridor looks like. Not a pretty sight.
The current Wellington plans include no PT corridor at all.
Without a high-quality parallel PT corridor, transport science tells us the road will inevitably disintegrate back into escalating congestion.
This is not just my opinion.
The Opus TN24 Report professionally modelled the effects of the proposed RONs projects, including Mt Victoria Tunnel duplication, and showed morning traffic volumes from the east rising to 154% of 2011 volumes by 2041 (appendix A, table 2, line 5). Unsurprisingly the congestion through the Mt Victoria Tunnel and along Ruahine Street remains at LOS E (“traffic operating at or near capacity, characterized by unstable flow, very low speeds, extremely limited maneuverability, and high driver frustration”). I’d welcome modelling that refutes these projections.
This is 2041. What will it look like in the year 2100… or 2200?
This doesn’t mean we shouldn’t be making road improvements. But doing this without incorporating supporting high-quality parallel PT means the road improvements are doomed to fail and can only be seen as stupid.
This is exactly what the NZTA are planning.
