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.
Design speed has a profound effect on our communities. Impacting far more than just safety, design speed influences urban design, walkability, public health, and even traffic lights.
Speed kills. That common expression illustrates a major concern with high-speed travel: a higher chance of death and serious injury when a crash occurs. But a higher risk of death is just one of many consequences of high-speed design. So far this series has covered what design speed is, how it is determined, and the direct impacts it has on street design. If speed is a drug, you can bet that it comes with side-effects; high-speed design has repercussions that spill over into all aspects of urban life.
Collision Severity
There is a universal relationship in all types of collisions: the faster the impact speed, the greater the chance of death or severe injury. Depending on the type of collision, there is also a noticeable “departure point” where, at impact speeds below the point, there is a good probability of a collision being relatively minor, and at impact speeds above the point, the chance of death or severe injury rises quickly.
For example, if you jump into water from a height of 30 feet (a high pool diving board), you’ll most likely be alright. A jump from 220 feet though (height of the Golden Gate bridge), and it’s almost certain you’ll be killed; the “departure point” where diving starts becoming dangerous lies somewhere just above 30 feet.
Research has shown that 9 out of 10 pedestrians hit by vehicles at 20 mph (30 km/h) survive, while at 40 mph (70 km/h) only 1 out of 10 will survive. In other words, 20 mph is the departure point above which getting hit by a car becomes increasingly lethal.
Here are some common “departure points” documented in design guidance, above which certain types of collisions become increasingly likely to kill or injure a road user:
- Pedestrian struck by vehicle: 20-30 km/h (15-20 mph)
- Vehicle strikes the side of another vehicle: 50 km/h (30 mph)
- Vehicle impacts another vehicle head-on: 70 km/h (45 mph)
If we seriously care about protecting the lives of the public, these numbers matter greatly. We don’t build 60 foot diving boards at public pools because we know the consequences would be lethal, so why are we comfortable building roads that place people walking next to vehicles going lethal speeds?
Cone of Vision
At higher speeds, you’re less likely to notice things at the side of the roadway. Even at speeds we would consider relatively low (50 km/h, 30 mph), a driver has almost no ability to pay attention to their most immediate surrounding environment. This is partially why suburban retail plazas rely on massive roadside signs to get the attention of drivers.
Transportation designers know this too, so they use different size signs for different roadway conditions. For example, the size for a speed limit sign on a freeway is twice that of one for a single lane conventional street.
Stopping Distance
We rely on our senses to navigate our environments, and when it comes to driving, our vision and hearing are the most important. When an unexpected event happens that we need to respond to, there is a delay in our response – first we must notice the issue, then make a snap decision, then carry out that action. This sequence of events takes between 1.5 and 2.5 seconds for most people.
If that response includes slamming on the brakes, the vehicle will still travel a certain distance before stopping. Stopping sight distance (SSD) is the total distance a vehicle travels before coming to a complete stop including the time it takes to detect a hazard, and it varies considerably with speed. At a design speed of 30 km/h, SSD is 35 metres. For 50 km/h that nearly doubles to 65 metres, and at 80 km/h it doubles again to 130 metres. The faster you’re going, the longer it takes to stop.
Unexpected events happen all the time: a child might run into the street, a cyclist could swerve to avoid a pothole, or another driver might run a red light. The speed you’re travelling when the unexpected happens could mean the difference between life and death.
The design speed of a street determines the design sight stopping distance, and a great amount of design flexibility is lost when the design SSD is higher. If a designer wants to add a pedestrian crossing to a street, for example, they must first confirm that the location chosen provides enough SSD for motorists coming from both directions. If the sight distance is insufficient for the design speed, it may not be possible to provide the crossing.
Noise
If you’ve ever spent time outside of a vehicle near a busy roadway, you’ll know the kind of impact that traffic noise has on urban environments. Higher vehicle speeds create more noise; on most urban roadways, a 10 km/h reduction in vehicle speeds yields a 40% decrease in noise levels.
Too much noise pollution hurts our health. More exposure to noise leads people to experience more stress and loss of sleep, and affects people’s ability to concentrate, and too much noise can even affect our ability to think creatively. Shopping streets that are too noisy because of higher traffic volumes and speeds may discourage shoppers from spending too much time there.
Traffic Lights and Stop Signs
Though it may seem counterintuitive, when speeds are higher, there is a greater need for “artificial” controls like stop signs and traffic signals to manage intersections for safety. On streets and at intersections where speeds are low, there is more ability to provide yield crossings, which are less complex, less expensive, and actually result in less delay. There’s a reason why the Netherlands, a country with a much better road safety record, has almost no stop signs and significantly less traffic lights per capita than Canada and the US.
Space for Other Modes
High-speed design requires a lot of space; you need wider lanes, wider medians, longer turn lanes, and wider clear zones. High-speed design also creates more pressure to separate slower motorists, so right turn lanes are added at every intersection and driveway, and lay-bys are added at bus stops, widening the roadway even further.
With all of this width allocated to motor vehicles, there is often little space left for pedestrians and cyclists. Transit suffers too, as buses are delayed when merging back into traffic from lay-bys.
Lower the design speed and you improve the conditions for walking and cycling. A 40 km/h urban street doesn’t need a centre median, wide lanes, or bus lay-bys. Clear zones are not even warranted at this speed because motorists are unlikely to be injured in a collision with a roadside obstacle. This allows more trees, benches, and other pedestrian-friendly amenities to be added.
Design speed has a profound effect on our communities. Impacting far more than just safety, design speed influences urban design, walkability, public health, and even traffic lights. Yet for years design speed has evaded the public eye, instead being treated as a purely technical matter. Are all these consequences really worthwhile amidst a climate emergency, rising inequality, and growing road safety movement?
All of this concern begs the question: what is the right speed to design for? We’ll talk about this in our next post.
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.
Beyond the Automobile 
A question always comes to my mind about statistics like this:
“Research has shown that 9 out of 10 pedestrians hit by vehicles at 20 mph (30 km/h) survive …”
I’m hoping you can clarify.
Does this mean a pedestrian is likely to survive being hit by a vehicle which *was* travelling at 30 km/h, or that the vehicle is *still* moving at 30 km/h at the moment of impact? (In other words, do the cited speeds indicate the speed *before* whatever inadequate braking the driver might have been able to do?)
I find it hard to believe my chances would be very good if I were struck by a 1000 kg lump of steel moving at 30 km/h.
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