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Mode Choice and Rational Commuting
Russell Degnan

Amongst the various comments about the proposed Metro line and the new station at Fishermans Bend, certain forms of analysis stood out, not for their accuracy, but their falling back on cliched understandings of travel, and the limitations of simple analysis in complex areas.

There are issues with the new proposal, mostly around the absence of long-term strategic planning in favour of project lists, which is not a new complaint.

There are fewer issues with a station at Montague, notwithstanding the commentary suggesting otherwise. Jason Murphy is normally very good, but his use of coverage diagrams merely highlights their limitations. Coverage is only as valuable as the service being connected to, the services that service connects to, their frequency and the line speed. Moreover, residential coverage is meaningless in the context of an area with several major destinations (the conference and exhibition centre, and South Wharf) and the expectations of significant future employment. What matters is the way that area connects as a destination. A connection to South Yarra and Southern Cross is potentially very useful; if debatably similar to the proposed (if unlikely) connection to Newport and Flinders Street.

The complaint of both Daniel Bowen and Tony Morton that residents would not use the station to connect from the light rail is in a similar vein: correct but basically irrelevant, if the stop is considered as a destination.

But they highlight a more general transport problem that is worth noting and explaining:

Public transport has big trade-offs for short trips

This often comes under the problem of the last mile, whereby a trip that ends at a transport hub needs a short connector that is hard to serve efficiently. But the problem of serving the last mile is true generally for short trips.

Consider someone within the residential area of the proposed Montague station, working in the CBD. In general, the potential user won't be either next to the residential station or working at a station in the loop. For the sake of argument we'll put them 400m from each.

As a walk, it is 3km into the centre of the CBD, or around 30 minutes. A trip to Docklands would be shorter, other parts of the CBD potentially longer.

As a train trip is is two walks of 400m (or 8 minutes), two trips through the station and onto underground platforms (2-4 minutes), waiting for the train (2-5 minutes), and a 2km trip on the train to Southern Cross (3 minutes). Somewhere between 15 and 20 minutes, most of which is spent walking and waiting.

On any transport, the walking distance can be shortened by reducing stop spacing. But here again, there is a trade-off in travel time as the stops add minutes, and the transport spends more time accelerating and decelerating at lower speeds. A tram with a 200m stopping distance would quarter the walk on average (2 minutes) but stop six times (5 minutes), and take 6 minutes to make the trip at the lower speed. Making the trip, including wait time, between 13 and 18 minutes.

In his piece on Fishermans Bend Alan Davies sourced a graphic showing walking lengths very substantially by mode in Melbourne. Most likely, this reflects two things: firstly, that stopping patterns are shorter for buses, and therefore the walk is naturally reduced; and secondly, that, although the environment has some effect on preferred walking distance, commuters are largely rational with respect to walking distance and time. Train journeys are longer, and walking is more efficient than the poor connections available in suburban Melbourne (on which, more later).

That being the case, the "rule of thumb" noted by Jarrett Walker of stop spacing between 400 and 800 metres is flawed. If we assume for rational commuters trying to minimise time, for a given trip length, there is an optimal stop spacing where walk time is offset by the speed of the transport. Because commuters go vastly different lengths this distance actually varies, and is often substantially longer than 800m. Though as a future post will show, this is complex; for large metropolitan areas, connections matter, a lot.

But on a short segment such as to Fishermans Bend, where most CBD/Docklands or local trips will be between one and four kilometres, there is no time advantage to having a heavy rail line with stops a kilometre apart through the area. The maximum possible trip is a mere 7km, which just barely goes past 1km as an optimal spacing. Stop spacing of a kilometre is double the optimal length for most of those trips. Yet on heavy-rail, any further shortening of those stops is impossible, and would both: effectively make the service a very expensive light-rail line; and were there a Newport connection, significantly slow suburban passengers.

The graphs below show the various trade-offs, although they slightly over-simplify the longer stopping lengths as at some point it becomes quicker to just walk, and wait-time is eliminated. [1] Notice too, that the optimal stopping distance is, as expected, between 400 and 800m for trips in this range.

There may be a future capacity issue in Fishermans Bend, given the projected population, and there would be a case for a station in Wirraway that connects to local transport in the event the South Morang-Newport connection occurred. But the future residents of the suburb will be much better served by efficient (and substantially cheaper) light-rail/tram lines. The vast majority of their trips (and the only ones reliably performed by public transport now) are too short to gain anything from services better designed for much longer journeys.

[1] Some assumptions need to be noted: the transport in question has a 1m/s2 acceleration and deceleration time, with intersections ignored (ie. light-rail, either grade separated or gated), a 40 second stop penalty, 5 minute waiting average, and a walking speed of 5km/h. It reflects averages; the transport time and waiting time will vary, obviously.

Planning 10th May, 2014 19:40:19   [#] 

Comments

Mode Choice and Rational Commuting
Hi Russell,

I *really* like those graphs on stop-spacing. You can be sure I will reference them in a blog post at some stage!

Thanks for the link to my piece. To make sure I understand your argument: you think the problem with a Fishermen's Bend/ Montague station is that heavy rail is just not suitable for short trips, ergo a stop near the city is wasteful?

I'm certainly not standing up for the fishermen's bend station, but does that not assume the purpose of inner city stations is assumed to be picking up people and dropping them in the city? Some inner city stations (I'm thinking South Yarra here) work as destinations in their own right for people from the outer suburbs.
Jason Murphy  21st May, 2014 12:17:34  

Mode Choice and Rational Commuting
Hi Jason, thanks, I have some more graphs like that coming, when I find a free moment.

I agree 100% with your last paragraph - the same could be said of Domain. I probably haven't made myself clear, but my argument was both that: Montague is a reasonable place for a destination stop, as it will connect to the tram and (supposedly) 40,000 jobs, and that it shouldn't be critiqued as a residential pick-up; and that the proposal by PTV (and others) to serve the precinct of Fishermans Bend with heavy rail (connecting through the CBD) is misguided because the trips are short and the right-of-way hella-spensive.

There is a broader point to be made about the need to use rights-of-way for their most efficient use and for heavy-rail to fit the role it is best suited for; but it can wait for another post.
Russ  21st May, 2014 21:28:24  


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