Balancing Mobility and Safety at Intersections
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Maximizing both the mobility and safety of road users at urban and suburban intersections is of utmost importance to city leaders and citizens today. Trends such as micromobility, connected and automated vehicles, and an explosion of available data, coupled with increasing numbers of bikes and pedestrians on our streets, result in both challenges and opportunities.
The increasing ability to provide intersection connectivity, edge computing and cloud storage, along with growing tool sets, such as Signal Performance Measures (SPM) and advanced video detection, provide new and exciting opportunities to traffic engineers. Possible combinations of Vision Zero intersection solutions, Near-Miss analyses, and the ability to make real-time operating decisions at our intersections can be overwhelming. Still, they must be embraced to ensure public officials are accountable to the traveling public.
This article will explore a historical perspective of managing mobility and safety at intersections, and will discuss examples of using advanced technology and data sources to maximize both in the state of Florida.
Finding the Balance
Operating a safe and efficient intersection ultimately entails creating a balance between safety and mobility. The safest intersection would be one without movement, and thus no opportunity for a collision. However, that would fail the mobility test.
At the other end of the scale, the greatest level of mobility would occur when everyone keeps moving with no delays. At a signalized intersection, that would not be very safe, as conflicting movements would quickly result in a crash.
So, what goes into establishing the right balance between the two?
The fact that we even get into a vehicle, or walk along a sidewalk, indicates that we accept some level of risk for the sake of transportation. Our transportation system is not guaranteed to be safe.
However, an overarching goal is to make the system reasonably safe by using design standards, adopting and enforcing the rules of the road, promoting vehicle features that enhance safety, and training road users on how to use the facilities. Or, as termed by many, the “three E’s” of traffic safety: engineering, education, and enforcement.
Consider how you make a left turn at a signalized intersection. At the simplest of intersections with no separate left-turn signals, you are free to proceed when your signal is green and you can clearly see that there is a gap in oncoming traffic sufficient to complete the turn. This scenario uses the highest level of driver judgment and increases the element of risk. Traffic engineers call this a permissive movement.
At some intersections, you might have a signal that allows you to turn left on a green arrow only, in what is called a protected movement. In this case, the opposing traffic stops, so you do not have to worry about finding a gap. There is very little judgment involved and, therefore, much lower risk. But you have to wait longer, impacting your mobility, as the arrow is green only a limited portion of the time.
In between these two extremes is what is called a protected/permissive turn. Drivers can make the left turn under the protected green arrow as well as during a permissive circular green or flashing yellow arrow indication. The protected green arrow helps move queued left-turning vehicles through the intersection, while the permissive indications allow turns when adequate gaps are available. This is a prime example of balancing risk with mobility.
We also create a balance between safety and mobility in our timing of the yellow change and red clearance intervals for each movement of the intersection. The yellow change is designed to provide notice that the green indication has ended and that a red indication is following; its duration is based on the speed of approaching vehicles and an assumed driver reaction time. The red clearance is designed to allow a vehicle entering the intersection at the last instant of yellow to clear the path of any conflicting traffic before they receive a green indication. For either the yellow or red intervals, too short a time would increase the risk of a crash, but too long a time would increase delays and reduce the intersection’s efficiency.
In years past, the focus of both safety and mobility in transportation was on vehicular traffic. We had become a “car-centric” society and, other than in downtown cores, pedestrians were ignored and bicycles were generally considered toys. We have all come to recognize and embrace the benefits of non-powered transportation, and providing for both pedestrians and bicyclists is part of traffic signal design and operations.
Pedestrian features at a signalized intersection not only advise when it is safest (and legal) to cross, but also provide sufficient time for a pedestrian to cross the street. Outside of urban core areas, pedestrian signals are typically activated by pushing a button on the street corner, though new technology can allow the passive detection of a waiting pedestrian. When activated, pedestrian signals provide Walk and flashing Don’t Walk intervals based on the street’s width and the adopted walking speed of a pedestrian, this often extends the movement beyond what is needed for motorized traffic. Again, a balancing of mobility and safety; motorists are slightly delayed to improve pedestrian safety and mobility.
Our current signal control technology allows us to vary our strategies by the time of day or traffic conditions. Overnight traffic demands are usually a lot less than during the day, so maybe we can make left turns permissive overnight, reducing delay while not significantly increasing risk. Perhaps pedestrian crossing times are increased at school crossings during the before and after-school crossing times. We can have adjacent signals working together on a common background cycle to allow for progression through a series of intersections, which improves both safety and mobility.
Toward the Future
We are on the cusp of significant advancements in signal control with the evolution of connected vehicles, and greatly enhanced computing and communications technology. Computers will take over much of the driving task, reducing the opportunity for human error and thus decreasing risk. And, by harmonizing flow through vehicle-to-vehicle communications, the efficiency of operations will improve as well.
Imagine if we could:
- Change left-turn modes between protected only, protected/permitted, or permissive. This would balance safety and mobility based on current sight distance limitations and the availability of sufficient gaps in oncoming traffic.
- Automatically extend the red clearance at an intersection to protect the safety on conflicting traffic patterns, based on predictive analytics that a vehicle will run the red light.
- Alert a driver of a pedestrian in the crosswalk to avoid a collision.
- Predict near misses between vehicles at intersections and change operating conditions in real time to avoid a crash.
It’s all coming! However, these advancements will not occur overnight. Many agencies experiment with evolving combinations of detection, analytics tools, communications, and business practices. Some combinations include a managed service concept, to ensure Vision Zero and congestion reduction goals can both be met.
Iteris is working in Florida to tackle an ever-growing number of fatal pedestrian crashes at signalized intersections, as high as any other part of the nation. The Florida Department of Transportation is piloting a near-miss system that will evaluate the risk of vehicle-pedestrian crashes at key intersections to apply solutions to avoid future crashes. In addition, the City of Lakeland is deploying an Intersection Collision Avoidance Safety Program (iCASP) that takes advantage of the City’s existing high-resolution traffic controllers and red-light camera system. Both of these systems are considered advanced vehicle detection systems that can detect the speed of approaching vehicles to determine their probability of stopping for a new red indication. When triggered, the systems could be programmed to hold conflicting movements until the violating vehicle has passed.
It can only get better from here! The balance between safety (through engineering, education, and enforcement) and efficient mobility will always be a feat. However, city officials and engineers can look toward automation, data availability, and advanced analytics solutions at our intersections to get a better bang for the buck and, most importantly, save lives.
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