Q&A: Vision Zero for safer roads, with Dr Suzy Charman

Dr Suzy Charman, executive director of the Road Safety Foundation in the UK, talks about the research that proves how cutting vehicle speeds and smarter infrastructure design can reduce death and serious injury on road networks – and the need to therefore challenge conventional approaches to scheme appraisals

Q: Vision Zero and the safe system approach are built around accommodating human fragility. What does research tell us about speed and survivability?

A: It’s a matter of physics and biomechanics. We know there is a clear link between driven speeds and survivability – it’s not all that popular to be talking about speed, but it’s fact.

Our recent research looks at the speeds where 10% of personal injury collisions are fatal or serious. The results show those 10% speeds much lower than you might imagine. For pedestrians and cyclists, it’s about 12mph. For motorcyclists, 20mph, and for car occupants in a side or head on collision, it’s about 30mph.

Enforcing those speeds would be quite challenging for a number of reasons. But moving existing speeds down even only a tiny bit has a big impact on death and serious injury. So that’s the job right now: trying to move those speeds down as much as we can to get that win, in terms of reducing death and serious injury.

Q: Why was a 10% risk threshold chosen?

A: We thought that 10% risk of being killed or seriously injured was analogous to energetically heading towards Vision Zero and a safe system. We’re calling on road authorities to think about what risk is acceptable, and to set their speed limits and enforcement strategies accordingly. It’s about being explicit about the trade-offs. When we allow higher speeds, we’re accepting higher levels of death and serious injury. That needs to be a conscious decision based on understanding the biomechanics, not just what feels comfortable, or what we’ve always done. 

Q: Speed reduction schemes often struggle to get approval because of economic appraisal. What’s going wrong?

A: What we typically do at the moment is we look at the safety impact of a scheme and weigh it up against the journey-time impact. When you then aggregate all the traffic that’s going a little slower, you end up with a huge journey time disbenefit. When we monetize it, it dwarfs the safety benefit. 

Now arguably, the Green Book [UK Government guidance] would suggest you don’t have to take account of a journey time disbenefit when your primary objective for a scheme is safety – saving lives. But if you feel you have to weigh it up, then I would also say we have to look at all of the cobenefits, which we rarely do very satisfactorily.
There are often lots of environmental benefits to slightly slower and smoother journeys – pollution, carbon use, fewer particulates being released from cars on braking. Then there is journey time reliability which improves with lower speed limits, which I feel is actually more important than journey time in and of itself. 
We’re trying to challenge the way we appraise schemes, to allow us to do the things we know will have a massive casualty reduction benefit, in part because we know there are other co-benefits.

Q: Beyond speed, what else needs to be at the heart of Vision Zero thinking?

A: We need to understand the humans that use the networks we’re building – not only in how they think, evaluate, perceive, and make lots of errors, but also from a human fragility perspective. 

We need to understand biomechanics, because we have to provide a system that is ultimately safe when a collision occurs, because they will always occur. That’s the fundamental principle of the safe system – we can’t eliminate human error, so we must design infrastructure that protects people when things go wrong.

Q: What gives you hope for Vision Zero?

A: We know what works. We have the evidence. We understand the physics. Cities like Helsinki have achieved zero fatalities for periods of time – it shows it’s possible, even if progress can be fragile.

The challenge isn’t technical knowledge – it’s having the courage to implement what we know works, even when it’s not popular. It’s about being honest about the trade-offs we’re making and putting human life at the centre of our decision-making, not journey time at any cost. If we can shift that mindset across the industry, Vision Zero becomes achievable.