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Traffic impact assessments: four brainworms that refuse to go away

How can we work, through transport impact assessments (TIAs), towards a better city? How can we do this when the inherited toolkit for TIAs is outdated, the City bureaucracy is stretched and design consultancies work for impatient developers for whom time really is money?


The Street Minds group did a great job unpacking these and other issues at the latest meeting hosted by Open Streets Cape Town. I was left, though, with an uneasy feeling. After almost thirty years as a transport planner it seems we are stuck, as Susan Handy has argued, on the same old roundabouts. Are we doomed to remain there or are we ready to move on?

This left me with four thoughts that are begging for more interrogation.

1.When we measure levels of service (LOS), what do we mean (and do we actually want to focus on it?)

Levels of Service

(From Maryland State Highway Administration)

The TIA process has at its core the assessment of traffic so as to make sure developments don’t detrimentally impact on existing movement patterns. Impact is measured through vehicle levels of service (LOS) (among other things). This idea of levels of service, which is widely used in South African transport planning, is rooted in the earliest freeway design manuals from the 1960s.

The LOS idea has unconstrained free-flowing vehicle movement as the ideal and everything else as some sort of failure. This dream of free-moving traffic may be appropriate for the freeway context but is this the metric by which we want to judge our city streets? Even aiming for some sort of more realistic LOS B or C for city streets in a very short peak period is problematic. Thinking about LOS ideals keeps our focus trained on the wrong sort of metric – comfort and alleviation of frustration for motorists only.

2. So what about LOS for pedestrians?

One counter to the focus on LOS for vehicles is to generate levels of service for pedestrians, but these, too are problematic. Instead of considering the multiple complexities of human movement, LOS for pedestrians treat humans as simply another ‘vehicle’ in urgent transit from A to B. Pedestrians though, unlike vehicles, move in far more complex, less constrained and multi-directional ways. Conscious human beings – unlike vehicles – come with a whole set of senses, vulnerabilities and needs. Designing for humans outside of vehicles requires design thinking well beyond the efficiency, density and comfort thinking embodied by LOS.

3. What should we be aiming for with TIAs?

It’s a well-established realisation in transport planning that adding road infrastructure is, at best, a short term “solution” to congestion. In the long term, additional road capacity simply exacerbates and fuels traffic growth by inducing traffic. Yet, as transport planners we operate with a sort of cognitive dissonance when it comes to traffic growth, knowing that adding road capacity is a short-term measure while expecting developers to contribute to new road infrastructure. Developers themselves often perpetuate the status quo, wanting to release bottlenecks near to their developments so as not to stimulate the ire of their tenants and give their developments a bad public rap. So we live in the realm of the short-term fix.

One way out of this is to require more sophisticated responses from developers and a practical possibility is the Travel Plan – an institutionalised, site-based travel demand strategy popular in the UK. But anything like this requires the City to really prioritise the long term over the short term and the greater good over the developers. Tricky even in a stable and well-resourced State.

4. What are we doing when we model traffic?

[vimeo 209719456 w=640 h=360]

Barbarossaplatz, Cologne – Traffic Simulation from heyne on Vimeo.

There is something  seductive about traffic simulations. Instead of being stuck in traffic, we are hovering above traffic and it’s an inspiring, creative space. This is very useful for designers. But I am deeply wary of these tools; they need to be handled with care. They create what Donna Haraway calls “the God-trick” and with it there is a risk that these tools disconnect the designer from the lived human experience and the traffic engineering exercise. It’s exacerbated by the traffic engineer’s focus on vehicles and the quantification of movement. It’s not that traffic engineering doesn’t need traffic simulations, but that they don’t go far enough. We live with all five senses and feet firmly planted on planet Earth and we need to design that way too. Not hovering above the traffic like a seagull.


[Thanks to the Street Minds group for a great discussion and to Marco Gerretto, Marcela Guerrero Casas and Brent Smith for edit suggestions.]


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