By Derek Jander, P.E.
My last article discussed the concept of Vision Zero, which is the name of the strategy that is used to inspire adaptions in our transportation network that work toward not just reducing, but having a goal to eliminate, all traffic fatalities and severe injuries for everyone. This includes, but is not limited to, vehicle drivers, pedestrians, and bicyclists. I also offered a limited introduction on a few key Vision Zero concepts related to roadway design that can be used to create safer, more forgiving streets for everyone. This article will provide a more in depth understanding of this roadway design concept.
For years, roadway design engineers, and I include myself in that cohort, designed roadways to be as “safe” as possible with the intent that by designing wide traffic lanes, wide shoulders, gentle curves, and clear roadsides, the as-built roadway would eliminate potential obstacles and objects that may be contacted should an errant vehicle run off the road. Roadway design engineers relied upon the American Association of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets to guide our safe roadway designs. Designing roads with wide lanes and wide shoulders and wide clear zones sounds great on paper, and these are important considerations when designing transportation facilities specifically geared for high-speed travel (think interstate highways). However, when it comes to the local street network, the network that provides direct access to our places of business and shopping and our schools, we have learned that maximizing the efficient through-put of motorized vehicles should not be the only guiding principle of local street network design. Higher speeds on streets is a fundamental factor in crash severity as my last article pointed by showing a pedestrian hit by a vehicle travelling at 20 MPH has a 90% survivability rate, whereas a pedestrian hit by a vehicle at 40 MPH has a 10% survival rate. With a greater mix of traffic types on our local streets these days, that is motorized vehicles next to pedestrians, bicyclists and scooter riders, and now e-bike riders, there is a warrant to consider the methods and means for managing or controlling motorized vehicle speed.
How do we do that? Fortunately, the most recent edition of AASHTO’s A Policy on Geometric Design of Highways and Streets has started to incorporate updated design guidance that provides greater flexibility in the street design based on the context of the street, for example if a street is in a rural town, suburban, or an urban core. Also, other design guidance has emerged that is, in some cases, being used instead of AASHTO’s publications because the other design guidance places more emphasis on streets for all users, not just motorized vehicles. Other options for street design guidance come from the National Association of City Transportation Officials (NACTO), which includes several publications related to a more shared street design experience. Just a few of the available publications include, the Urban Street Design Guide, the Urban Bikeway Design Guide, and Designing for All Ages & Abilities. NACTO’s website at nacto.org provides links to these publications, as well as other publications, that provides some of the publication information free for review and provides some great detail on a shared street design network, which includes vehicle speed reduction strategies.
Examples of some of the speed reduction street design strategies include:
- Lane Width Reductions: Customary lane width for streets is frequently 12-ft wide. This width makes sense for a high-speed highway facility with frequent truck traffic, but for a local street, more narrow lane widths usually make more sense because narrower streets promote slower driving. There are complex reasons for this, but a narrower traffic lane communicates, almost instinctively, that a slower speed is necessary for the street.
- Pinchpoint: This strategy restricts the lane width on both side of the curb line for a short segment of length to create a narrower traffic lane for a short length. Drivers will tend to slow their speed on an approach to a narrowing of the traffic lane, so pinchpoints are sometimes considered traffic calming devices. Pinchpoints also provide extra sidewalk width to help create shorter pedestrian crossings.
- Median: This is an island in the middle of the street that creates a “pinchpoint” in the roadway and communicates an effect similar to the concept of a narrower lane. As a bonus, a median provides a point of pedestrian refuge and reduces crossing distances for pedestrians crossing the street.
- Chicane: These are curb extensions that alternate on both sides of the street to make the street appear as though there are left and right turns on the roadway. The street becomes less “straight,” and drivers tend to slow down when they see curves on the street.
- Diverter: These serve to break up the “through” movement of vehicles within an intersection while still providing access for pedestrians and bicyclists. A diverter effectively runs diagonally across the intersection so that vehicles cannot drive straight through and forces vehicles to turn and consequently slow down.
These are but a handful of examples of ways that smart design strategies can help vehicle drivers intuitively slow and down and make the local street network a safer place for all road users. Other strategies also exist. In the meantime, the next time you are driving down the street and you see some new curb lines, medians, or barriers in place on the street, please think of these as safety strategies to reduce the severity of accidents. Accidents will happen as drivers and pedestrians and bicycle riders are all human and will not avoid contact under all circumstances, but with some smart design strategies, we can reduce the severity of those accidents. And cutting the severity of accidents is something we can all advocate for.
Also, engineering standards and practices are dynamic and can and do change with a growing basis of understanding on safety. Investigating a roadway incident is not a one shoe fits all consideration. However, with reasonable design choices based upon location and use, our roadways should become a safer environment where injuries are reduced and lives are saved.
If you have a matter in litigation that involves Roadway/Highway safety, the experts at CESI can help. Contact us for a no obligation case evaluation.