Traffic Engineering Myths Revealed

We are finally emerging from the InterState era.  This was the long period where the vision of the ideal road was the limited access freeway – a road designed specifically to move as many cars as possible as quickly as possible, with wide lanes and soft curves, while eliminating potential distractions such as stores or traffic lights or any other method of travel by foot or bike.  The InterState was about moving vehicles.  People were only important as the occupants of those vehicles.

The InterState era was also a time when what every self-respecting traffic engineer really wanted to do was create highways or at least car-centric designs.  Quiet residential roads or people-focused plaza were boring – the money and glory was in becoming another Robert Moses: the man who transformed New York with his highways and bridges, a master builder.

I’m not sure what to call the era which we are now entering – “people-centric design” perhaps – but I know that one of the problems we face are the myths created during the InterState era that allowed traffic engineers to believe that car-centric design was not only good but inevitable.  Many engineers have come to understand that different times require different visions, different values, and different designs.  In fact, current research shows that many of the old beliefs were simply wrong.  But far too many still cling to outmoded practices that put them at odds with growing demands for more livable streets.

Citizen groups need to know the cultural assumptions that traffic engineers were trained to use in the past when doing road design, and the alternative reality that current transportation research has revealed.

1) Level Of Service

Old Assumption:  The key measurement for a good road is its “level of service” which is graded from A to F and functionally defined as the speed with which a road is cleared of traffic.

Reality:  The proper measurement of a transportation system is not how many vehicles it moves or how fast they go or even how open the path is before them.  In a city, speed simply measures how fast a car gets to the next red light.  A better metric is the amount of through-put – the number of vehicles that pass through a stretch of road and intersections, which we now know is maximized by slow but steady traffic flows created by controlling turns, positioning parking, and well-timed traffic lights.  But in the final analysis, the most important measurement is not about vehicle movement at all, it is about the impact of the transportation system on people’s lives – getting a positive balance between the benefits and the costs in terms of people’s health, safety, ability to socialize with their neighbors, and get around.  For example:  giving priority to transit, walking, and bicycling moves more people more cost-effectively than any other mode — and sometimes requires that car traffic backs up a bit.

2) Overbuilding

Old Assumption:  When designing a road, always assume that people will exceed legal limits and therefore always build in the capability of handling speeds and traffic volumes of at least 25% over expectations.

Reality:  The structure of the road shapes the behavior of those who use it.  If you design a road to handle high speeds, drivers will use it.  If you want people to go slowly you have to design a road that encourages and even requires it.  For example: wide travel lanes and gentle curves were intended to make roads safer, but instead have been shown to increase average speed which, ironically, reduces safety.  In fact, recent research shows that reducing city lanes from the old 12-to-15 foot width down to 11, 10, or even 9 feet reduces the number of accidents, injuries, and fatalities.

3)  Designing For Extremes

Old Assumption:  A road should be designed so that it can handle the most extreme situations, such as a fire truck racing to a house or a double-hung trailer truck trying to get around a corner.

Reality:  Designing for extreme situations is an invitation for ordinary users to act extremely.  Instead, design a road to handle ordinary use, but build in the flexibility to handle the occasional extreme demand.  For example, “tight corners” and “curb extensions” at intersections slow down normal traffic, but keep parked cars away from the intersection which allows long emergency vehicles to make the turn by running over the corner sidewalk if needed.

4)  The Basis for Road Standards

Old Assumption:  National standards are scientifically developed to provide the “one correct answer” to traffic design situations.

Reality:  National standards are actually “guidelines” that provide significant flexibility so that traffic engineers can design roads to meet the needs and desires of the community and the location — an approach called “context sensitive” design that accepts that road design is as much an art or craft as a science, and that accepts that it is legitimate (even necessary) for “laypeople” to participate in design efforts.  Traffic lanes do not need to be 15 or even 12 feet wide; research shows that in urban areas lanes as narrow as 9.5 feet wide are just as safe – and in some ways safer – than wider lanes while still allowing high levels of  traffic “throughput.”

5) The InterState Ideal

Old Assumption:  The Interstate Highway is the model of best-practice road design.

Reality:  The Interstate is a unique type of road built for a unique purpose – long distance travel between widely separated destinations.   The vast majority of roads serve radically different purposes and require radically different designs.  In fact, the Interstate is an aberration, not the norm, and a danger when misapplied.  A highway is NOT a livable street.

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