Measuring accessibility: PTAL and what to use elsewhere

A Public Transport Accessibility Level tells you how much public transport sits within walking distance of a point and how often it runs. It tells you nothing about where that transport goes. Two sites can share a PTAL of 4 while one is twenty minutes from a town centre and the other is an hour, and the index cannot tell them apart.

Understanding that gap is the practical skill. In London the score is unavoidable, because parking policy is written around it. Outside London there is no equivalent, and the accessibility case has to be built from components. Either way, the index is a starting point and not an argument.

How the index is built

The calculation works outward from a point, usually the centre of a site or a grid cell.

First, identify public transport access points within the walk catchment. Bus stops count within roughly 640 metres and rail, underground and light rail stations within roughly 960 metres, which correspond to eight and twelve minutes at the assumed walking speed. Anything beyond those distances is excluded entirely.

Second, work out a total access time for each route serving those points. That is the walk time, plus a scheduled waiting time derived from the service frequency in the assessment period, plus a reliability allowance which is larger for buses than for rail.

Third, convert each total access time into an equivalent doorstep frequency, then sum them, weighting the best route at full value and the others at half. The result is an accessibility index, which is banded into levels from 0, meaning effectively no service, through 1a, 1b, 2 and upwards to 6b in the busiest central locations.

The assessment period matters and is easy to overlook. A site near a station served heavily in the morning peak and sparsely at other times will score better than the lived experience suggests.

What the score leaves out

Four things, and each of them is where a considered transport case earns its keep.

Destination. The index counts services, not journeys. A high frequency orbital bus route that does not reach the hospital, the college or the employment area is worth the same to the index as one that does.

Journey time. Frequency and proximity say nothing about how long it takes to get anywhere. A site with a PTAL of 3 next to a fast rail line can outperform a site with a PTAL of 5 served only by slow local buses.

Quality of the walk. The catchment is a network distance, but the index does not care whether the route crosses a dual carriageway, runs through an unlit underpass or has no dropped kerbs. For anyone with a pushchair or a mobility impairment, that is the whole question.

Crowding and span of service. A route that is full at 08:15 or stops running at 19:00 counts the same as one that does not.

Better evidence in London

The index stays in the assessment because policy requires it, but it should be one page rather than the whole chapter.

Time mapping outputs, which show the area reachable within a stated travel time from the site, deal directly with the destination problem. A map showing that 45 minutes by public transport reaches the town centre, two hospitals and a major employment site is more persuasive than a band letter.

Add the walking route audit. Photograph the route from the site to each access point, note crossing points, lighting, gradients and severance, and say what the scheme will improve. Where a section 106 contribution or a section 278 works package can fix a poor crossing, that is a concrete accessibility gain rather than an assertion.

Then connect it to parking. London Plan parking maxima are set by PTAL band and location, so the score feeds directly into what is acceptable. Where you are proposing more or less than the standard suggests, the case for evidencing parking above or below the standard needs observed demand rather than policy arithmetic, and a parking survey carried out to a recognised methodology is what supplies it.

Accessibility outside London

There is no national index, and inventing one for a single application rarely helps. Build the case from parts instead.

Walking and cycling isochrones from the site, using the actual network rather than radii, showing what is reachable in five, ten and fifteen minutes on foot and in ten and twenty minutes by cycle. Name the destinations reached rather than the area covered.

Timetable evidence for every bus route within a defensible walk distance, showing frequency by time of day, first and last service, and Sunday operation. Frequency alone is misleading where the last bus is at 18:30.

Rail evidence where relevant: service frequency, journey time to the principal destinations, and the state of the interchange.

Published journey time statistics, which give travel times to employment, education, health and food services by mode for each small area, and let you compare the site against the district average in a way an officer can check.

And local policy. Many adopted plans define a walkable distance to a service centre or apply a settlement hierarchy. Test the site against the plan's own definitions before importing anything from elsewhere.

Some authorities outside London have adopted their own accessibility index or bought modelling software that produces something similar. Where one exists, use it, and use it in the form the authority publishes rather than recalculating it yourself. An officer comparing your output against theirs and finding a difference will spend the rest of the assessment looking for the reason.

Tying accessibility to what you are asking for

Accessibility evidence exists to support a proposition, and the proposition is usually one of three things: that the site is a sustainable location for this quantum of development, that the parking proposed is appropriate, or that the travel plan measures are proportionate.

Say which, early, and shape the evidence to it. A Transport Assessment that presents four pages of accessibility analysis and never links it back to the parking ratio has wasted the effort. A Travel Plan with targets that ignore the fact that the last bus is at half past six will not survive first contact with a highway officer.

One honest caveat. Accessibility evidence persuades on marginal sites and rarely rescues an isolated one. Where the nearest service is an hourly bus that stops in the early evening, the better argument is usually about what the scheme will provide, whether that is a service contribution, a new stop, or a cycle link, rather than about how the existing position should be characterised.

Enter the site postcode on PlanScope to see which transport documents your scheme is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Councils expect transport evidence to be prepared by a suitably qualified transport planner, and this article is general guidance rather than legal advice.