There are those, still many in number I suspect, who continue to understand urban design as being mostly, if not entirely, about how things look. Such people are prone to coming up with such priceless pronouncements as: 'Just let me traffic-design this junction, then you can urban-design it,' as if the latter is little more than choosing whether the guard-rails should be stainless steel or wrought iron.

By way of proving that urban design does (and indeed must) reach much further, my focus this month is on public transport accessibility levels (PTALs).

PTALs, let's face it, aren't at all about how nice places look (even though I'll admit that the typical PTAL 'heat maps' that show which areas are better or worse off from a public transport point of view can add a splash of colour to otherwise dour transport policy documents). Despite this, they can have an unexpected and very significant influence on the nature and form of urban development. I know, because I've been there.

One of the projects I'm working on at the moment is the comprehensive redevelopment of a large housing estate in London. There are, as you can probably imagine, a number of major issues to be negotiated, including the usual suspects of cost, phasing, tenure mix and doing right by existing residents. But, long in the tooth as I am, the extent to which these big issues are critically influenced by transport, and specifically public transport accessibility considerations, has frankly amazed me.

In a way, of course, it's highly gratifying: transport people love to find that the world does, in fact, revolve about them. But, while basking in our supremacy, we would do very well to take to heart the words of wisdom that Peter Parker (aka Spider-Man) received from his Uncle Ben: 'Remember: with great power comes great responsibility'.

We need to take our transport decisions in the context of a full understanding of their potential non-transport implications. Or look at it another way: transport projects should support places, not impose upon them.

Returning to this estate regeneration project, there are two particular ways in which the matter of public transport accessibility (PTA) is proving to be highly influential. The first concerns the relationship between development density and PTA.

We're probably all reasonably familiar with the Transport Development Area/Zone concept - essentially shorthand for the idea of massing development around powerful public transport nodes/interchanges. Well, the same principle can apply much more generally.

Many local authorities, for instance, set their parking standards in relation to PTALs, with less parking to be provided where better PTA makes this reasonable and practicable. In the light of the London Plan, the client authority for this project has also determined to tie allowable residential densities to PTALs.

While not a problem in principle, we have encountered serious practical ramifications. In short, the redevelopment initiative needs to substantially increase residential densities within the study area in order to allow the construction of sufficient new private-for-sale units to cover the cost of reproviding the same number of affordable dwellings as presently exist.

There is strong support within the authority for this approach but its adoption of rigid density-PTAL relationships directly challenges the authority's ability to pursue it.

One particularly intriguing aspect of this problem is that this policy stance was at least partly driven, and remains partly sustained, by the desire of some politicians to resist increased residential densities in their wards, a ticket they perceive to be a vote-winner locally.

These brave civic leaders cannily latched on to PTALs as a tool they could use to support their populist approach, with low PTALs enabling them to justify their resistance. But in their desire to maintain low PTALs for this reason they tend to resist moves to improve public transport for their constituents. This, surely, is a type of 'perverse incentive' that PPG13 never envisaged!

We move on now to the second big PTAL-related conundrum. In order for the necessary new densities to become achievable under the density-PTAL rules, a significant boost to PTALs in the study area is required.

For this we're dependent on the delivery of a major public transport initiative that's been on the drawing board for some time. Naturally, there are a variety of concerns about if and when this initiative will actually be constructed but what's proving probably the biggest headache in planning terms is the fact that the 'official' methodology for measuring PTALs simply doesn't reflect the increase in actual PTA that all parties agree will follow if the scheme is implemented. A brief explanation for the uninitiated follows.

The official PTAL methodology scores accessibility essentially on the number and frequency of public transport services available and the distance from the stops/stations at which those services can be accessed. It takes no account of where those services run to and from or of journey times between origins and destinations and only reflects the quality of vehicle in the broadest fashion (train is more attractive than bus).

Frankly, every practitioner knows this a really blunt tool: a decent first stab and relatively simple to use but only vaguely reflective of how punters perceive the quality of public transport services on offer. But since it's the only recognised tool in the box, we're obliged to use it.

In summary, therefore, we've found that the very basis of the design of a huge urban regeneration initiative hinges in large part on PTALs. What's more, the initiative is in jeopardy because (a) the way we're required to measure PTALs is flawed and (b) the way some want to use PTALs runs contrary to the reason we're measuring them in the first place! Bizarre, but true, and surely a very clear example of the importance of the inter-relationship of transport and urban design at the highest level.