Let's make transportation infrastructure more like a roundabout - just not round.
Roundabouts are designed for people, which sounds obvious on the surface; however, understanding the ‘why’ roundabouts work opens the door to designing other parts of transportation better by using the same principles that have led to their success.
Roundabouts create an environment where people can make good decisions more often and more easily, while being more forgiving when mistakes happen. That principle is the heart of successful transportation design: creating a transportation network designed for those who use it.
One of the biggest challenges transportation users face, especially drivers, is balancing competing demands for their attention, whether that is looking for an upcoming turn, reading a sign, listening to a child in the back seat, noticing a pedestrian, or the flashing lights of an emergency vehicle. The more information, complexity, and decisions people must process to navigate the road, the greater the potential for confusion, stress, and mistakes. As drivers, we often do things to reduce our cognitive load, even a little. For example, have you ever turned down your music before looking for an upcoming turn, as if the extra sound makes it harder to find what you are looking for? In some ways, it does.
For some people, roundabouts can initially feel confusing or intimidating, especially if they are unfamiliar with how they work or associate them with difficult experiences at older rotaries and traffic circles. Yet modern roundabouts simplify the decisions people need to make and when they need to make them, even in the busiest places. This matters because not everyone is the same: people have different experience levels, physical or cognitive capabilities, familiarity with the area, and personalities that shape how they approach situations and process information. When a design systematically simplifies information and decisions, it better accommodates differences in experience, ability, confidence, and familiarity. This is critical at intersections where roads with different purposes and characteristics must share the same footprint and benefit from predictable, orderly choices that reduce uncertainty, indecision, and apprehension. Let’s look at how modern roundabout design creates an environment suited to good decision-making and how those same principles can be applied throughout transportation design.
Simplify what people need to decide.
Simplifying decisions is a core benefit of roundabouts. A primary way roundabouts simplify decisions is by reducing vehicle speeds. Roundabouts are designed so that controlling vehicle speed is a primary concern where people’s paths cross, regardless of the mode of transportation. Slow speeds are easier to track and predict, making it easier to decide when to go and when to stop, whether you’re a driver, cyclist, or pedestrian. When traveling more slowly, drivers can take in and process more information, allowing them to recognize and react appropriately to whatever situation they may encounter. Increasing adoption of traffic calming and speed management practices in environments that require more decision-making from drivers, bicyclists, and pedestrians allows everyone to take in the information they need and understand it correctly.
Roundabouts use their shape to guide what people can and should do as they move through them. For example, splitter islands control the immediate space around the roundabout. They help enforce speeds, limit access from nearby driveways, and reduce the likelihood of wrong-way turns. Transportation infrastructure benefits when it is self-explaining and self-enforcing. When people instinctively understand what is expected, they can focus on the decisions that matter.
Separate where people need to decide.
Roundabouts break complex decisions into manageable pieces. A great example is the process a driver goes through when passing through the intersection. All drivers, regardless of destination, generally undergo the same process. At the point of entry, the driver’s primary decision-making question is relatively simple: is there conflicting traffic that requires me to yield? Once circulating, the driver’s attention can shift primarily toward maintaining their path and recognizing their intended exit. Transportation design can benefit from separating complex decisions into simpler choices at all scales. For example, median U-turn (MUT) intersections simplify left-turn movements by replacing the direct left turn with a sequence of simpler movements using a downstream U-turn, while two-stage crosswalks simplify pedestrian crossings by allowing people to cross only one direction of traffic at a time.
Drivers, cyclists, and pedestrians interact at intentionally located points that reduce competing demands on attention. For example, crosswalks are typically set back from the yield line, separating the pedestrian crossing decision from the decision to enter the roundabout. Bicyclists ahead of the intersection can decide, based on their comfort level, whether to navigate the intersection with vehicle traffic or enter the shared-use path and cross via a separated multimodal facility. This merge-and-diverge point occurs where vehicle and bicycle speeds are relatively similar, making the interaction more predictable and easier to manage. Infrastructure design benefits when interactions between different modes occur where people can clearly see, understand, and respond to one another, rather than relying on added measures to compensate for an inherently difficult interaction.
Reduce the consequences of mistakes.
People inevitably make mistakes; they are only human, and the human body is vulnerable. Roundabouts create an environment where mistakes are less likely to lead to crashes, and when crashes do occur, they are less likely to be serious. Mistakes are less likely to lead to crashes because the slow-speed environment gives people more time to recognize and react to each other, and vehicles can stop more easily over a shorter distance. Crashes are less likely to be severe because they involve less energy. Crash energy increases with the square of speed, so a vehicle at 40 MPH has roughly four times the kinetic energy of the same vehicle traveling at 20 MPH. By keeping speeds lower within the intersection, less energy is involved when crashes occur, reducing the likelihood that a mistake results in a serious injury.
That is what it means to make transportation infrastructure more like a roundabout, without making it round: Design around how people actually make decisions. Give them the right information when they need it, without asking them to process everything at once. Give them enough time to understand what comes next, and most importantly, design the system so that when someone inevitably gets something wrong, the consequences don’t have to be severe.
For media inquiries or further information, please contact:
Jessica Ciccarelli
Greenman-Pedersen, Inc.
813.838.3202
jciccarelli@gpinet.com
