The question of walking distance in transit is much bigger than it seems. A huge range of consequential decisions -- including stop spacing, network structure, travel time, reliability standards, frequency and even mode choice -- depend on assumptions about how far customers will be willing to walk. The same issue also governs the amount of money an agency will have to spend on predictably low-ridership services that exist purely for social-service or "equity" reasons.
Yesterday I received an email asking about how walking distance standards vary around the world. I don't know the whole world, but in the countries I've worked in (US, Canada, Australia, New Zealand) the view is pretty consistent:
- If you have to choose a single walking distance standard for all situations, the most commonly cited standard is 400m or 1/4 mi. Europe tends to be comfortable with slightly longer distances.
- However, people walk further to faster services. (Rail advocates are more likely to phrase this as "people walk further to rail".) This doesn't have to be a sociological or humanistic debate, though urbanists often frame it that way. If you are a rational and informed actor seeking to minimize travel time, it often makes sense to walk more than 400m to a rapid transit station rather than wait for a bus to cover such a short distance.
- Although the common standard is 400m or 1/4 mi, we all know that this cannot possibly be a hard boundary. It makes no sense to assume that if you live 395m from a bus stop you'll be totally happy to walk that distance while your neighbor, who lives 405m from the same stop, will be totally unwilling to. Obviously, the relationship between distance and willingness to walk is a continuous curve without sharp breaks. This has to be said because our language often forces us to create the illusion of sharp breaks, e.g. when we say something like "people are generally willing to walk up to 400m to transit."
Finally, it's remarkably hard to sift data into a form that produces unequivocal guidance on the question. For example, the leading US guide on transit planning, the Transit Capacity and Quality of Service Manual, offers only this:
(Source: TCQSM Chapter 3, Appendix A, p. 3-93. Discussion and version in US units is on p. 3-9.)
This survey-based graph shows the breakdown of local bus passengers by the distance they walked to get to the service. As you'd expect, few people walk more than 200m in downtown Washington, DC because in such a densely served area, few people would need to. In low-density Calgary, at the opposite extreme, many people have to walk fairly long distances.
But extrapolating opinions from behavior is a tricky business. It's hard to reason from how far people walk to how far they're willing to walk. To do that, you'd have to determine whether each rider would be willing to walk further than he actually has to walk. You'd also have to speculate about each rider's available options. If 1/10 of Calgary's bus riders walk 600m or more, does that mean they're willing to? Or does it mean that these people are so lacking in good alternatives that they feel forced to walk that far? (The difference between "high income" and "low income" Washington DC suggests that range of options does have something to do with it.) Sociologists and demographers can have a field day parsing this question, but they're unlikely to come up with an answer of such statistical certainty that it definitively sweeps the question aside.
So we approximate. We generally assume that 400m is a rough upper bound for slow local-stop service, and that for rapid-transit (usually rail) we can expect people to walk up to 1000m or so.
But when we try to apply these rules of thumb, we hit another hard issue (or at least we do if we're willing to acknowledge it). Are we talking about true walking distance, or just air distance? Over and over, in transit studies, you'll see circles around bus stops being used to indicate the potential market area, as though everyone within 400m air distance is within 400m walk distance.
Remember this graphic?
In both images, the red dot is a transit stop and the red circle is an air-distance radius. If you draw 400m circles around stops based on the assumption of a 400m walking distance, you're implying that the whole circle is within walking distance. In fact, even with the near-perfect pedestrian grid in the right-hand image, the area within 400m walk (outline in blue) is only 64% of the red air-distance circle. With an obstructed suburban network like the left-hand image, it can be less than 30%.
Obviously, the market area around each stop should really be defined by the walkable area, which requires a knowledge of the local pedestrian network. That requires a complete GIS database of every walkable link in the community -- an extremely detailed task that few jurisdictions have been willing to attempt until recently. Even in Canberra, Australia, which is known in the business for the extreme richness of off-street pedestrian connections, no reliable database of them was available for modelling purposes as recently as last year.
Still, if you don't have such a database, it should be easy to adjust the walking distance standard to reflect the problem. If you know you have a good street grid, then you can just adjust the radius to reflect the area within 400m walk. In the right-hand image above, do the math and you'll figure out that if the radius of the red circle is 400m, then a circle whose area is the same as that of the blue diamond -- the actual area in walking distance -- would have a radius of 319m. So if you want to roughly model the actual radius that arises from a 400m walking distance, and you have a well-connected street grid, draw a circle 319m in radius. That doesn't give you the correct boundaries of the area -- it's a circle rather than a square -- but it's a far better approximation than just drawing a 400m circle. I have never actually seen this done, and I'm not sure why.
One reason might be that secretly, we transit planners all want people to walk further. After all, most transit planners don't want to just passively respond to current behavior. If they did, they'd all be highway engineers. Most transit planners believe in the importance of shifting behavior in more sustainable directions, and see both transit ridership and walking as deserving encouragement through intervention. They are also aware of the public health benefits of walking.
But we have a more vivid motive to encourage walking. The nature of the transit product is such that if we could stop less often, assuming longer walk distances, we could achieve both better running times and reduced operating cost; the latter could be reinvested as higher frequency. So the two most fundamental determinants of transit travel time -- running time and frequency -- both depend on our assumption about walking distance.
With such basic things at stake, it's understandable that planners are always looking for ways to push walking distance wider. That may be the real reason that generations of planners have chosen to approximate a 400m walk with a 400m circle, even though every pedestrian knows how absurd that is.
I prefer to just have the argument in simpler terms. In Canberra, we pushed the walking distance standard from 400m to 500m, not because people were calling us demanding to walk further, but rather because we looked at how much more frequency and speed we would achieve, and the ridership that could attract, and decided that 100m of radius was a small price to pay for such benefits. It comes back to that graph near the top of this post, showing how far people walk to transit in different cities. There's no definitive authority for a 400m standard as opposed to 300m or 500m or even 600m. Yes, if you pick a bigger radius you'll lose riders from the outer edges of the radius, but on the other hand, you may buy so much travel time and frequency that your ridership goes up. As with everying else in transit, it depends on what you're trying to do.