Coming to an Intersection Near You
SPUI helps control heavy traffic
Diamonds may be forever but not necessarily when it comes to traffic interchanges.
In Utah, Florida and Michigan, highway engineers are replacing the traditional diamond interchange with a single-point urban interchange, known as SPUI. SPUIs can handle heavier flows of traffic--between 1,500 and 5,500 vehicles per hour--by allowing cars diagonal from each other to make left turns at the same time, and are especially effective where traffic is equally heavy in both directions. "Operationally, they are more simple," says Mack Christensen, of the Utah department of transportation. "You eliminate one signal, which allows you to control traffic at one point. Therefore, you reduce the number of conflicts and phases."
SPUIs also take up less space than a traditional diamond interchange: Diamond interchanges require about 1,200 feet of right-of-way space, while SPUIs only need 250 feet.
But there are some problems with the new configuration. Drivers may be confused when they make a left turn and find that cars coming from the opposite direction are also making a left turn--on their right side. A SPUI is also more expensive to build than a diamond interchange--usually by 30 percent to 50 percent, depending on terrain and existing infrastructure. The greater cost of a SPUI can sometimes be offset by not having to purchase as much land since its requires less space.