Changing directions in today’s traffic control

If you drive in Florida, you’ve undoubtedly noticed an increasing number of non-traditional intersections. Though traffic signals and stop signs are still the most common type of intersection traffic control in use today, alternative intersections are quickly gaining popularity throughout the state. Modern roundabouts, in particular, have seen a tremendous uptick in recent years. So why the shift in intersection traffic control methodologies?

As a confluence of vehicles, bicycles, and pedestrians, roadway intersections have the greatest impact on the general safety and efficiency of a given transportation network. For these reasons, safety and efficiency, alternative intersections are becoming more common. From a safety perspective, by minimizing the number of conflict points and lowering the relative speeds of conflicting vehicles, you can decrease the chances of crashes with serious injury. Also, by reducing delays and increasing “green time” at intersections you can improve the intersections efficiency, thus reducing traffic congestion. The roadway designers’ goal is to maximize both safety and efficiency, and there can be multiple ways to do that.

Roundabouts are not a new concept. They are widely used in other parts of the world, and more common in other states within the US. In more recent history, some of the kinks with older “traffic circles” have been worked out lending to improved design and better functionality, hence today’s common reference as “modern roundabouts”.

While some remain skeptical, the use of roundabouts is relatively simple. All traffic moves in a counterclockwise circle (and yes, they flow clockwise in the United Kingdom). Approaching vehicles must yield to vehicles already in the circle. You enter the circle when there is gap and simply exit in your desired direction. Multiple lane roundabouts will typically have sufficient signing and marking at the approach, instructing drivers which lane they should be in depending on their desired destination. Once accustomed to them, many folks prefer them over traditional stop sign or traffic signal-controlled intersections.

The safety benefit of roundabouts is two-fold. First, by forcing all vehicles to slow down, the severity of collision and chance of serious injury is greatly reduced (remember Newton’s second law?). Secondly, by having all traffic move in a similar direction, the total number of conflict points is reduced. Similarly, pedestrians are crossing where vehicles are traveling at slower speeds. “Splitter Islands” on the approaches also give pedestrians a refuge, allowing them to cross only one direction of traffic at a time.

While there are decided benefits, there are also a couple drawbacks to roundabouts. One is the increased area needed for the circle. In urban situations or intersections with very tight right-of-way there may not physically be enough room. Roundabouts are also not typically used, at least in Florida, on six lane or higher roadways due to the increased complexity of movements.

Roundabouts are not the only alternative intersections gaining popularity, but most work under the same general premise…get rid of the left turn. Left turns take a lot of “green time” away from other movements. Think about it, to turn left at a traditional intersection, you need to cross the opposing through traffic and both lanes of cross traffic. That means three out of the four major movements have to stop for you to turn left. With a finite amount of green time available in any cycle, signal timing becomes a question of which movement needs it the worst. Effective alternative intersections are those that are able to “get rid of the left”. In roundabouts, everyone makes a right to get in, and a right to get out. The counter-clockwise flow is essentially a continuous left. Some examples of other common and successful alternative intersections are displayed on the right.

There is not a “one size fits all” solution for every intersection. To help engineers decide which is best the Florida Department of Transportation has developed the ICE (Intersection Control Evaluation) analysis method. Finding the right solution is not always an easy answer but the good news is… we have options!
For more information contact Ryan Bell, PE, PTOE at mkt@johnsoneng.com.