How is Design Speed Determined?


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 Blacka 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.


When engineers and policymakers decide a speed to design for, they are also pre-determining the character of the roadway, who it’s going to be built for, and who will be excluded.

In the first post of this series, we introduced the concept of design speed and how it influences the shape and feel of streets, and ultimately how fast we feel comfortable driving. This begs the question, how is design speed actually decided? 

Overdesign

Design speed is not the same as the posted speed of a roadway. While the legal posted speed can be changed with a bylaw and a few signs, design speed is determined at the onset and is much harder to change once established, making it even more important to get right. If a design speed is much higher than a posted speed, drivers may feel comfortable or even compelled to speed.

One of the most common approaches to determining the design speed is to “overdesign” the roadway, by selecting a design speed that is higher than the expected posted speed. The concept of overdesigning things is common in the engineering field, and usually there’s good rationale for it. For example, if you’re designing an elevator, you would design the steel cables that support it for the maximum number of occupants plus a considerable amount of extra weight. The same thing applies when you’re designing a bridge. This helps account for the variability in how things will be used in reality.

When it comes to road design, the “safety factor” mentality assumes that the road should be designed for a faster speed than you would expect a motorist to travel, so that a motorist who is speeding is less likely to run off the road or be seriously injured. This means that for a new arterial street with a posted speed limit of 60 km/h, the design speed could be 70 or 80 km/h, resulting in wider, straighter lanes and clearer sightlines with fewer obstructions.

A street posted at 35 mph (60 km/h) that contains design features of a higher-speed street (Source: Wikipedia)

What this approach overlooks though, is the feedback loop that it could create. As we discussed in our last post, higher design speeds can encourage motorists to drive faster. Engineers will later collect speed data on the road and notice that speeding is common, which actually creates pressure to increase the speed limit to match those higher speeds. This feedback loop does not exist in other engineering examples; an overdesigned elevator, for example, doesn’t encourage more people to pile into the elevator.

It is a self-fulfilling prophecy: experience shows that when design speeds are higher, people will drive faster, which necessitates higher design speeds to accommodate the people who drive faster.

Precedent & Consistency

Once a roadway has been designed to a certain speed, it is very difficult to change it. When a road is rebuilt, engineering guidance typically recommends that the design speed be selected based on the operating speed of the roadway, which if course is influenced by the existing design speed. This means that if the street is operating at 70 km/h, that would typically become the design speed – even if the posted or desired speed is 50 km/h. This puts the onus on individual engineers to push for the more progressive approach of changing it, and those engineers even risk the threat of lawsuits or accusations of professional malpractice for going against the grain.

Maintaining consistency in the roadway cross section is also highly emphasized in engineering guidance. Of course, there is reason for this – if the design of a roadway changes abruptly without warning, it could lead to driver confusion and more collisions. But there are many valid reasons to change the design speed of a roadway along its route, such as when it passes through a school zone or through a town centre. 

Policy & Priorities: Speed, Volume, Safety

Above all, policy is what shapes our transportation network. The engineering field is ultimately in service to the public, and so public policy is what rules in determining design speed. Policies are both written and unwritten; for example, the policy to prioritize fast travel for motorists in road design underpins almost every street design decision but is rarely stated outright. As of late, road safety and Vision Zero have emerged as public priorities and have begun to shape policy too.

Traditionally in road construction projects, engineers consider vehicle design speed, traffic volume, and overall safety – in that order. First, the desired design speed is established through iterations based on policy, constraints, property, and existing context. Then, traffic volumes determine the vehicular capacity of the roadway that will be designed for. Once these parameters are established, remaining efforts are spent trying to make the space safe for all users.

The challenge of this approach is that when you start with the assumption of high vehicle speeds, you limit your options for providing a safe travel environment. As discussed in our first post, high design speeds necessitate larger clear zones, more gradual curves, and wider lanes. If a slower design speed were selected from the onset, there would be less need for countermeasures like guardrails to be used later in the process.

There are examples of policy measures that have truly improved urban streets for the better. The Dutch Sustainable Safety policy was developed in the 1990’s and has dramatically improved the safety of Dutch streets. This is one of many examples of an Injury Minimization approach to roadway design; that is, where safety is the first factor considered, with speed and volume being secondary. We’ll talk more about these policies later in the series. 

What’s Missing? Context

For new roads, policy governs design speed. For reconstructions, precedent prevails. Overarching these decisions are the principles of maximizing speed and consistency.

A highway running through a school zone, with no changes to the design except a sign. (Source: Strong Towns / Google Maps)

But what about areas where slower speeds might actually be desired? There are many examples of such contexts:

  • A retail area with on-street parking and frequent driveways
  • A school zone or a college campus
  • A transit station with significant pedestrian and curbside activity
  • A town centre or historic downtown
  • A mixed-use area that aims to be pedestrian-friendly with streetscaping and wide sidewalks

When speed and consistency govern the design of roads, we lose sight of the multi-purpose nature of streets and the importance of context. When engineers and policymakers decide a speed to design for, they are also pre-determining the character of the roadway, who it’s going to be built for, and who will be excluded.

A highway running through a town centre, recently redesigned with traffic calming features to respond to the local context.

Flip this equation upside down and you realize that, used effectively, street design can help reinforce the desired context of the roadway, by sending subtle messages to drivers about how fast they should be going and how alert they should be of other road users. There are many good reasons to achieve harmony between context and design speed, and we’ll discuss those more in the 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.

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