RM 1431 Diverging Diamond Intersection
TxDOT is making intersection improvements to Ranch to Market Road 1431/University Boulevard at I-35 in Round Rock.
Improvements include the following:
- Replace the traditional intersection with a diverging diamond intersection (DDI)
- Construct a southbound collector-distributor road, or frontage road bypass road
- Make ramp improvements
- Build additional sidewalks to accommodate pedestrians and bicyclists
A DDI is a new intersection design that addresses congestion by allowing vehicles to travel more quickly through an intersection. It increases traffic flow by temporarily shifting traffic to the left side of the road, allowing through-traffic and left-turning traffic to proceed through the intersection simultaneously. This eliminates the need for left-turn arrows, which allows more flexibility addressing current and future traffic volumes. Read the fact sheet and view the diagram to learn more.
Construction on the $6.6 million project began in October 2014 and is expected to be complete in spring 2016. The contractor is Capital Excavation Company of Austin.
Similar to the design of a conventional diamond intersection, the diverging diamond intersection differs in the way the left and through-traffic navigates on the mainlanes. The purpose of this innovative design is to accommodate turning movements onto and from RM 1431/University Drive, while eliminating the need for a left-turn bay and signal phase at the signalized intersections. The design increases safety by reducing the number of potential conflict points between cross-traffic and left-turn movements.
In addition, drivers on the I-35 frontage roads will no longer be allowed to drive straight through the traffic signal at RM 1431/University Boulevard. They will have to turn left or right at the intersection or use the frontage road bypass lanes to continue north and south. This means drivers are able to bypass the intersection without stopping at a traffic signal. The northbound bypass lane is already in place, and the southbound bypass lane is included as part of the improvements.
How diverging diamond intersections work
A DDI creates a diamond-shaped pattern in the intersection and temporarily shifts traffic to the left side of the roadway prior to entering the intersection.
How it works:
- Two phase traffic signals are installed at the roadway crossover points.
- Once on the left side of the road, drivers can freely turn left, rather than waiting for oncoming traffic to clear or for a left-turn signal. Or, drivers can continue straight and will switch back to the right side of the roadway once they’ve cleared the intersection.
- U-turn lanes are also included in a DDI, making left-turns from the cross-streets safer and more efficient.
- Through-traffic on the frontage road bypasses the intersection via a bypass lane and proceeds straight through the cross-street intersection, allowing drivers to avoid the traffic signal.
This innovative design is particularly effective at locations that have high volumes of left-turn traffic. These relatively low-cost improvements work by reducing potential crash points at intersections and by allowing more cars to move through an intersection. This means a reduction in delays and travel time.
Other benefits of a DDI include:
- Motorists are able to bypass the intersection without stopping at a traffic signal
- Improved travel time because additional "green time" at traffic signals allow more vehicles to pass through the intersection
- Additional sidewalks increase safety and better accommodate bicyclists and pedestrians