Designing for Speed is a weekly multi-part series exploring how speed is factored into the design of our streets, how it influences safety, and how it ultimately shapes our communities. This series is co-authored with Dustin Black, a design engineer working at the intersection of transportation, land use, and the built environment. Join the Beyond the Automobile mailing list to stay up to date with the series.
Safe Systems determines the right speed to design for by considering the vulnerability of humans, so that if a collision occurs, it’s much less likely to be severe.
So far this series has covered why design speed is important, how it is determined, and some of the negative consequences of high-speed design. This all begs the question: what is the right speed to design for? To help answer this, we first look to one of the world’s leading world safety programs, the Safe Systems approach.
Originating in the Netherlands and Sweden in the early 1990s and stemming from the successful safety philosophies of the aviation and petroleum industries, the Safe Systems approach considers the interrelating factors that lead to road fatalities and serious injuries. Rather than seeing collisions as unavoidable “accidents”, responsibility for safety is shared by both humans and the road operators.
Perhaps most importantly, Safe Systems aims to create a traffic system that is as safe as possible for all users, where “the road and the vehicle protect you and those around you against major traffic hazards”. The philosophy is catching on around the world under the Vision Zero banner, but in the Netherlands and other successful countries, this program has gone far beyond a vision statement. The initial implementation of Sustainable Safety, the Dutch version of the safe systems approach, yielded a 4-to-1 cost benefit ratio, saving an estimated 1600-1700 lives over 10 years.
Since speed is a major factor on both the likelihood and severity of collisions, Safe Systems takes a strict approach to managing it. Maximum allowable speeds for streets are based on principles of biomechanics, ensuring that the speed of the roadway reflects peoples’ and vehicles’ abilities to withstand impacts, so that “the free flow speed is safe in the event of an incident”.
Let’s consider that sentence with a more local example. Imagine a suburban arterial with a design speed of 80 km/h with a painted bike lane on the side of the road. If an incident were to occur where a motorist swerved into the bike lane and struck a cyclist, will that cyclist be able to withstand the impact of a car travelling 80 km/h? Obviously the answer is no. A Safe System approach would correct this vulnerability by either significantly lowering the speed of the motorist to make a collision with a cyclist less severe, or separating the cyclist from the road with a bicycle path in the boulevard.
Here’s what the Dutch Sustainable Safety approach looks like in implementation:
30 km/h is the maximum design speed for streets where traffic is mixed with vulnerable road users, either midblock or at intersections (including bike lanes). At higher speeds, pedestrians and cyclists should be physically separated from motorists. Heavy traffic calming is applied to achieve this speed, with lots of speed humps and typically a brick roadway surface. This is the standard approach used for local streets, one exception being bicycle streets, which accommodate local traffic for cars but act as a through route for cyclists.
At 50 km/h, conflicts between motorists and pedestrians or cyclists should be prevented entirely. Midblock, pedestrians and cyclists are physically separated from motorists, and at intersections they are given dedicated signal phases. Note that this is the highest design speed where no clear zone is required, since at this speed, a curb is typically enough to keep a car on the roadway, and if a motorist does hit a hard object at the side of the road, they are unlikely to be seriously injured. Lanes are likely to be narrow, and roadside elements like trees and poles are placed intentionally close to the roadway to keep drivers’ speeds low.
At 60 km/h, no side (right-angle) conflicts between motor vehicles are permitted, so left-turning movements at intersections are given a dedicated signal phase. A clear zone of at least 2.5 m should be provided at the roadside, so if any trees, sidewalks, or bicycle paths are present, they must be set back at least this far from the roadway. 60 km/h is the typical speed for a rural roadway that is not a major traffic route.
At 80 km/h, head-on collisions between motorists should be prevented by adding a centre median, and the clear zone should be at least 6 m. If pedestrian and bicycle infrastructure is present and if this separation cannot be achieved, a barrier should be placed between the two. It’s more likely that pedestrian and bicycle facilities won’t be present at all on streets this fast, as these users are more likely to be given a convenient parallel route that is more pleasant than riding alongside high speed traffic (see the filtered permeability concept). Roadways like this are typically only found outside of urban areas, used to connect destinations over long distances.
Safe Systems is all in the name: it is a reconfiguring of the entire traffic system to be safe. Rather than responding to “collision hotspots” with targeted countermeasures, it considers the fundamental conditions that create potential for harm, and pre-emptively avoids those conditions through selecting the most appropriate design speed for the context. Although Sweden and the Netherlands pioneered this approach, they should not remain the sole beneficiaries. Fortunately, there is exciting progress happening on the Safe Systems front in the US and Canada, and we’ll delve into that in our next and final post of the series.
Designing for Speed will continue next week. Join the Beyond the Automobile mailing list to stay up to date with the series. Special thanks to Dustin Black for collaborating on this post. Dustin is a design engineer working at the intersection of transportation, land use, and the built environment. Follow him on Twitter @EngineerDustin.
Don’t forget to check out the other posts in the Designing for Speed series below.