On the network topology of spines

This post was originally published on Sam’s blog.

ECan recently released their draft long-term public transport plan for Christchurch and Timaru. On the other side of the globe, Jarrett Walker and co are proposing a new bus network for Dublin. The proposed Dublin network centres around the concept of a ‘spine‘ – a collection of frequent bus routes that use the same corridor in the central city, turning that corridor into a very frequent transit line.

The proposed Christchurch network consists of frequent ‘core’ routes and less frequent ‘connector’ and ‘cross-city’ routes. ECan’s definition of frequent is a bus every 10-15 minutes, with the idea being that you can just turn up to one of these bus stops without needing to consult timetabling material, knowing that you won’t have to wait long.

Here’s a sketch of what they’re roughly proposing, in diagrammatical form:

Their network has a radial topology, with 9 lines radiating out from the central city and one orbital line. New lines (with my own unofficial labelling) are Magenta, Brown, Green and Red. Under this network, a lot of Christchurch gets better service to the CBD (see Chris Morahan’s model). However, could we combine some of these frequent routes into common rapid transit corridors? Below is an attempt:

Under this altered network, we have two main lines – A (North/South) and B (East/West) – that branch into individual lines after travelling along common corridors (orange is lonely because I couldn’t figure out if it would be an A or a B). This would mean the Papanui to Moorhouse and Riccarton to CBD lines would be incredibly frequent, with a bus every 4 minute-ish. I’ve also shown heavy rail for a potential connection point at Moorhouse. It should be noted that the spines aren’t just collections of bus routes travelling along the same road – they are intelligently timetabled frequent routes, so that there’s a bus every 3ish minutes, 7am-7pm every day.

Is this better? I don’t think so. The extra frequent coverage under ECan’s plan is hard to argue with, and I doubt the benefits of a couple of very rapid corridors outweigh that. Also, the Christchurch City Council is putting a fair amount of effort into promoting the central city as an opportunity for development, and, to me, a rapid-spine based network may undermine that effort by encouraging [transit-oriented] development away from the CBD along Papanui and Riccarton roads. The coverage based network being proposed would likely do more for promoting development in the CBD, as evidenced by Chris’ analysis demonstrating that more people can reach it in less time.

Additionally, wayfinding can help promote some rapid corridors that exist under their plan anyway. For example, plinths like these above would be well suited outside Westfield Riccarton and at a Moorhouse Ave stop, where buses will likely be every ~2-4 mins at peak times under their network. This sort of signage would ensure people view the core network as a cohesive whole and help cement main corridors such as Riccarton – CDB and south Colombo St as rapid transit routes that can be travelled on without pre-planning.