Intersections are designed to ensure the safe and efficient travel of drivers, pedestrians and bicyclists.
The concept of an intersection is simple; it is an at-grade junction of two or more roadways. They can be controlled by many devices: lights, signs, striping or lane design. Some intersections have no controls at all. Regardless of design and controls, one thing is certain – the solution to managing traffic and keeping users safe around the clock is complex.
Alternative intersection designs are often considered when conventional designs do not allow for adequate traffic flow or safety improvements. These designs often help reduce the number of conflict points, resulting in increased driver and pedestrian safety. The animated graphics below help explain a few of the lesser-used intersection varieties and we examine some of the differences between them.
Reduces conflict points by more than 50 percent and increases throughput by 30 percent.
An RCUT, sometimes referred to as a superstreet or J-turn intersection, does not allow left turns or through movements from the side street. Instead, drivers entering the intersection from a side street are forced to turn right on to the main road and perform a U-turn at a one-way median opening. Motorists traveling on the main roadway can make left and right turns or proceed straight through like normal.
RCUTs help improve safety by reducing the number of conflict points (areas where vehicles are likely to come in contact with one another) from 32 at a conventional intersection to 14. They also eliminate the potential for far-side right-angle (T-bone) crashes. RCUTs are also more efficient. In a sense, they operate like two one-way streets. There are fewer interruptions to traffic flow which allows more vehicles through the intersection. They also decrease wait times for vehicles at the intersection.
RCUTs can have up to a 30 percent increase in throughput (the number of vehicles going through the intersection).
One example of this is the Saratoga Street and Highway 23 intersection in the City of Marshall, Minnesota. The Minnesota Department of Transportation (MnDOT) and the City ended up choosing an RCUT intersection design because severe right angle crashes had been occurring at the intersection. A traffic study found drivers had a difficult time judging the speed of traffic along the highway before entering the roadway.
Traffic volumes at the intersection weren’t high enough to warrant using a traffic signal or roundabout, making the RCUT the ideal solution.
To address non-motorized multimodal users, a trail bridge was constructed over Highway 23 so residents and students could safely cross to access schools, businesses, and services.
The RCUT should be considered for minor road intersections with four-lane divided highway corridor projects where there is a sufficient volume on the minor road.
Reduces conflict points by 50 percent and reduces the number of traffic signal phases incurred.
Similar to the RCUT, the MUT reduces conflict points for motorists traveling through an intersection. However, a median U-turn alters drivers’ left-turn ability on both the primary roadway and side street while allowing both main and side streets to make the through movement. At intersections with less side street through movements, another form of the MUT intersection completely closes the main crossing and makes all drivers trying to get to or stay on the side street go through the U-turn. A median U-turn, known as a “Michigan Left,” has been in use since the 1960s.
Because left turns are considered among the highest risk turns in an intersection, the MUT reduces total conflict points by 50 percent. Instead of turning left at the main intersection, drivers on the primary roadway proceed through where they make a U-turn at a downstream median opening, followed by a right turn onto their intended roadway.
Though median U-turns are typically used along corridors, they can be used at interchanges and isolated intersections as well. They are usually best suited for high capacity intersections where medians are wide enough to comfortably allow the U-turn. The median area itself can be used as a refuge, keeping pedestrians safer.
MUTs can reduce the number of traffic signal phases at an intersection. According to the Federal Highway Administration, by reducing the number of signal phases, median U-turns increase vehicle capacity through the intersection by as much as 20-50 percent.
Manages high volume intersections with fewer conflict points.
Like the RCUT and MUT intersections, the DLT, also known as a continuous flow intersection (CFI), helps to eliminate congestion caused by drivers making left turns.
At a DLT, left-turning drivers must cross over the opposing through movement at a location several hundred feet prior to the major intersection. This crossover is typically signal controlled. It helps eliminate a left-turn signal phase at the major intersection since opposing left turns and through movements are all made in the same signal phase for the main street.
DLTs also reduce the number of conflict points in an intersection as well as creating more separation between vehicles.
A DLT can be built in a partial form, where only the major road has the crossovers prior to the main intersection and the left-turn movements for the minor road take place at the main intersection as normal. Or, it can be built in a full form, where all approaches have the crossovers for the left turns prior to the main intersection.
Related Content: This Intersection Can Help You Solve Traffic Congestion
DLT intersections typically work well in signalized urban corridors where there are heavy through and left movements on the major road or both the major and minor roads. It is a good alternative to the very costly grade-separated interchange option.
SEH recently conducted a corridor concept design and operations analysis study on behalf of the City of Centennial, Colorado. Through this study, we created alternatives to improve traffic congestion along a 2.5-mile long section of the City’s Arapahoe Road. The road is currently a built-up, urban, six-lane facility with dual lefts at many of the intersections. SEH evaluated six intersection designs to accommodate 2035 traffic projections, improving Arapahoe Road mobility and keeping existing access. The study recommended partial DLTs at three intersections, a conventional intersection at one, and a modified version of a MUT with roundabouts on a minor road to allow U-turns at the other two. This layout derived from the study worked well as it provided more traffic signal green time for the massive volume on the major road while still allowing full access to/from the minor road.
In a green T intersection, one direction on the major road has a continuous flow of traffic, while the other direction is controlled by a signal.
As with the other alternative intersection designs, green T intersections help to reduce crashes by eliminating conflict points. Angle crashes at T intersections are some of the most severe accidents occurring at intersections. This option reduces the probability of drivers being t-boned when traveling into and through the intersection.
Drivers along the top part of the T (major road) continue through the intersection, while drivers approaching from the stem (minor road) enter the main roadway through a dedicated lane, without impeding on the drivers already in the top of the T lane.
These intersection designs only apply to three-leg intersections with the best results coming from sites where there is high volume on the major road and high volume left turns from the minor road.
Every intersection is unique and requires careful professional analysis to determine the best solution. This chart serves as a general guide to help you decide between four alternative intersection designs.
The importance of considering alternative intersection designsAs traffic congestion builds in cities and towns across the U.S., it becomes more challenging to usher large volumes of vehicles and pedestrians through our intersections. Wait times get longer and crash potential increases. If we take a step back and examine different solutions, we can make a difference.
Scott Hotchkin, PE*, is a roadway designer who understands that sometimes the most effective way forward is along a path less traveled. Contact Scott