Saturday, September 27, 2008

The Pedshed Series

or how to get the patronage gains of 10 extra stations without building them

Most people with a choice will only walk a certain distance to public transport. The generally accepted maximum is 800 metres/10 minutes to 'high quality' public transport (eg railways or busways) or 400 metres/5 minutes to 'regular' public transport (eg local bus routes).

The area surrounding a station within 800 metres or ten minutes walk is within its service area or pedshed. Since people are more concerned about time rather than distance, the service area of a station can be increased if overall origin to platform walking speed is increased.

Because walking speeds are a matter for personal discretion, the only way to do this is to eliminate all obstacles between station platforms, origins and destinations. The most significant of these obstacles are those where the passenger is forced to wait or is moving much slower than regular walking speed. Examples of obstacles may include unresponsive pedestrian crossings (or none at all), indirect detours, inconvenient entrances/exits and bottlenecks near ticket barriers.

The diagram below shows how a station's service area can be increased by improving access speed.

Because the increase in area is proportional to the square of the radius, a even small change could easily increase a station's potential catchment population by 10 or 20 percent. If pedshed enhancing measures were applied to all stations in a 200-station network, it's not unreasonable to expect patronage gains equivalent to building several new stations.

Let us get some more detailed figures on this.

800 metres in 10 minutes assumes a walking speed of 4.8 km/h.

Suppose we were able to speed station access times by 60 seconds. Instead of passengers being able to walk 800 metres in 10 minutes, they can now walk 880 metres in 10 minutes.

The pedshed areas are as follows:

800 metre pedshed: 3.14*(800*800) or 2 009 600 m2

880 metre pedshed: 3.14*(880*880) or 2 431 616 m2

This is an increase in service area of 21%. Patronage increase might not be quite this high (those 10 min away might be less likely than those 5 min away to use it) but even if half or three quarters the service area increase it is still substantial.

It works in reverse as well. Removing a subway or zebra crossing in favour of a signalised pedestrian crossing (for example) reduces access speeds and therefore a station's pedshed. In the example above, a minute's delay reduces the pedshed to 720 metres and coverage area to 1 627 776 m2 or a 19% reduction.

The other effect with signalised crossings is that waiting times vary, meaning greater variability in origin to platform times. Walking time via subway or zebra crossing is constant and predictable but becomes variable when they must encounter pedestrian signals (especially those on 90 second cycles). Assuming two crossings, worst case variabilty can be 3 minutes, which is a significant proportion of the ten minutes access time allowed. The effect is that passengers must build 'fat' into their schedule (by leaving early), further reducing origin to platform travel speeds.

A program to widen station pedsheds would require a range of measures of varying cost. These vary from subway or elevated walkway construction (most expensive) to altering pedestrian cycles, removing roundabouts, installing median strips or painting zebra crossings. However such measures are desirable if public transport is to operate at peak performance, delivering the lowest possible (and most predictable) door to door travel times.


Dave said...

Nice post. This should be copied or linked from TT, and your posts are still showing only "untitled", unfortunately - I just clicked through hoping something new was here!

One thing you didn't elaborate on was platform access itself (in the case of heavy or some light rail). Sometimes you basically have to walk the length of the platform to even get in - an extra 150-200m or so, which for most of us is an extra couple of minutes.

I'm wondering if with myki, in which you're penalised for not tagging in/out, whether you could have gates (with scanner) at both platform ends, not just the current single entrance at one end or centre. This would, for approx one third to half of approaching pax, reduce access time.

Vic Rail (Riccardo) said...

Remember too, a pedshed is based on the station's entrances if we are talking rail. SY is a good example, all the entrances at the Toorak Rd end, none at the Melbourne High end.

The boys don't care, but you have lost the opportunity to use the bridge to Cremorne as part of the pedshed.

Calembeena said...

I do care, Riccardo

Anonymous said...

As someone who regularly stands at Moonee Ponds junction diagonally opposite my tram stop waiting for the lights to change only to see a tram pull up at the stop unload/load and leave. I would also like to suggest that pedestrian traffic light sequences are one manor in which the pedshed can be improved. If we use your rule of thumb 10 minute limit on pedshed for every minute you stand at a traffic light we are losing 80m of effective distance. Although the biggest offender in this regard are Trams it also affects mnay of the major railway stations as well.