We often use 1/4 mile (400 meters for my SI-using allies) as the walk-shed for transit. This is too short. See e.g. these graphics from the Snelling Arterial BRT study, which draws radii around stops. 1/4 mile does not even get you from one end of Rosedale Mall (which isn’t even the biggest Mall in the Twin Cities) to the other, and many people make a full circuit, on two floors, inside the mall, on foot. If we have nice enough environments, we should expect people to walk a 1/2 mile to 1 mile with no problem, shopping mall developers do, and they are far more mercenary than the public sector.
A longer assumed walk-shed has many advantages. It allows us to increase spacing between stops, which increases running speed, which makes transit more attractive for those already on-board. We always trade-off running time for access time (higher access time for lower running time, e.g. when we space stops farther apart.)
Jarrett Walker at Human Transit, discusses the issue and notes that in many urban areas there is no need to walk farther than 400 m, so we don’t know what people would do. He also notes the difference between radiuses and network distances.
Schlossberg and Agrawal also discuss this in: How Far, By Which Route, and Why? A Spatial Analysis of Pedestrian Preference. They find the average pedestrian trip to a rail station was 0.47 miles (nearly 800 m). Guerra, Cervero, and Tischler ask “The Half-Mile Circle: Does It Best Represent Transit Station Catchments?” and argue it is useful (and a slightly better predictor) for the residence end of trips, though shorter distances (1/4 mile) at the work-end makes are slightly better predictors.
In short, I believe people will walk longer than we typically credit if we can make decent walkable urban environments, environments which lead people to under-estimate the actual time involved (as the saying goes: time flies when you are having fun) because their mind is not on how awful the walk is, but about how interesting the environment is.
David Levinson, Transportist 
It is more than 400 meters for buses and more than 800 for rail in most cases…..sometimes it can be less by the way….if you go back to the origin of the 400 meters it came from a study done with seniors in the 1970s.
The attractiveness of the transit service plays a big role on willingness to walk to use this service. High frequency service and comfortable one attracts people from bigger catchment areas. Of course the distribution of the land use around station plays a role as well as street connectivity etc. The direction where this transit service will take you, number of transfers, in vehicle time, amount of walking at the other end of the trip, income of the person or the neighborhood, and trip purpose. In short service area is a little more complex than applying a rule of thumb. The pleasant walk can play a role but not as big as the other stuff I mentioned in here.
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In my dissertation, I found that people walked a longer distance to transit, in my research the BART system, than they did to shopping. This was based on surveys in six shopping centers and at two BART stations. It was tough to sort out the distances travelled to bus stops.
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Regardless of distance, it’s nonsensical to use crow-flies distances in suburban environments like this, especially with barriers here like major freeways, megablocks and subdivisions with few entrances. Given how easy it is to do this with GIS, there really shouldn’t be any good excuse not to, unless the goal is to boost forecasts.
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And the operative point: “if we have nice enough environments”. In most US “urban” environments, we do not have environments which are conducive to walking. Furthermore, it only takes one significant obstacle to deter someone from walking to a transit stop at all.
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Steve Mouzon’s Walk Appeal, loosely based on the transect model, is a great illustration of this. http://www.originalgreen.org/blog/walk-appeal.html
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