Transit Accessibility, TOD and PTAL

The workhorse of transit planning is the station catchment: the area from which people will realistically walk to a stop. Rules of thumb put it at about 400 m (a 5-minute walk) for a bus stop and 800 m (a 10-minute walk) for a rail or metro station - people will walk further for a faster, more reliable service.

As in the walkability lesson, the honest catchment is a network service area, not a circle. An 800 m radius circle around a metro station routinely overstates the walkable catchment by a third or more, because rail lines, arterial roads and gated layouts sever the approach. The gap between the circle and the network catchment is exactly the population a project will claim to serve but cannot - which is why the network version is the one that belongs in an evidence-based transit study.

Distance to a stop is only half the story - a stop with a bus every hour is not the same as one with a train every three minutes. PTAL - Public Transport Accessibility Level - is the best-known index that combines both. Developed for London and now used widely as a template, it works in outline like this: from a point, find the nearby stops within walking distance; for each, convert the walk time plus half the average wait (the service interval) into a total access time; combine these into an Accessibility Index, then band the score into levels (commonly 1 = very poor to 6 = excellent, sometimes shown as 1a to 6b).

The insight PTAL captures is that access is walk time and wait time together. A location with two frequent bus routes 300 m away can outscore one right next to an infrequent stop. PTAL is a template, not an Indian standard - you adapt the walk speeds, thresholds and frequencies to local conditions - but the logic transfers directly and is straightforward to build in GIS.

If you can measure access, you can plan form around it. Transit-Oriented Development (TOD) is the principle that the highest densities, the greatest mix of uses, and the least car dependence belong inside the walkable catchment of high-quality transit - and that the built form should taper as access falls off toward the edge. India's national and state TOD policies apply exactly this idea, concentrating FSI and mixed use in an influence zone around metro and rail corridors.

GIS is how TOD stops being a diagram and becomes a plan. Compute the network catchment of each station; overlay current density, population and land use; and you can see immediately where reality already matches the principle (dense, mixed, walkable) and where a station sits marooned in low-rise sprawl or industrial land - the sites where the policy, and the value, are unrealised. The catchment map is the TOD brief.

You can build all of this on open data. OpenStreetMap supplies the walking network and, often, the stop and station locations; the network catchment comes from the same service-area tools you used for walkability, and population comes from the Census or a gridded set like WorldPop or GHSL. Frequency is the missing ingredient in many Indian cities - where a published, machine-readable timetable exists you can fold it in; where it does not, you use surveyed or assumed service intervals and state the assumption.

The cautions are familiar and real. Network and stop data may be incomplete; assumed frequencies can be optimistic; and accessibility is not the same as usage - a well-scored location can still be poorly used if the service is unreliable, unsafe after dark, or unaffordable. A PTAL-style score is a strong, comparable measure of supply; pair it with observation and ridership before you treat it as the whole truth.