Innovative Intersections

Intersections are crucial to a street’s performance; they greatly influence the road’s speed, safety, cost, and efficiency. Accommodating turns directly affects safety and efficiency, making left turns the key design factor in intersection improvement. Traditional left-turn lanes are not always able to adequately fix congestion problems at some intersections.

Many recently developed innovative intersection designs provide alternatives for accommodating left-turning vehicles.1 Many incorporate elements that seem similar to interchanges, but their at-grade design saves the cost of constructing overpasses. Some designs may also deliberately reduce average vehicle speeds while serving more vehicles and shortening travel times through the intersection and along the corridor.

Innovative intersection designs work best on major streets in suburban and growing outlying areas that have higher speeds and higher volumes.

How Will This Help?

• The design diverts left turns away from the main intersection and allows more green time for through traffic.
• It reduces the number of vehicles and/or the number of conflicting movements using the main intersection.
  • This provides for simpler and more efficient signalization, shorter cycle lengths, fewer conflict points, shorter delays, and improved traffic flow.2, 3
• Design options include:
  • A two-stage left turn. Before the intersection, vehicles turn left onto a road that is parallel to their initial road; they travel toward their desired road and turn left while the traffic on the main road has a green signal.
  • A right turn followed by a U-turn. These are typically for traffic from minor cross streets; all vehicles are required to turn right at the intersection with the major street. Vehicles that wish to travel in the opposite direction can make a U-turn through the median 500 to 1,000 feet away and join the major street traveling in the desired direction.
  • An adjacent minor roadway to handle turning movements. A separate road away from the intersection can be used to route left-turning traffic and simplify the signal system.

The design details are determined by a process described in a recent Federal Highway Administration report.2

The steps involve:

1. Establishing project objectives,
2. Assessing pedestrian activities and conflicts,
3. Assessing the available right-of-way,
4. Evaluating the access needs,
5. Estimating capacity and traffic volume, and
6. Conducting computerized simulation models to test viable alternatives.

Common innovative intersection designs include:

• Restricted-crossing U-turns direct the left-turning vehicles to make a U-turn at an opening in the median after they pass through the main intersection.
• Quadrant intersections direct left turns onto a connector road before the main intersection and guide the vehicles onto the intersecting roadway.
• Continuous-flow intersections separate the left turns onto a segment that is parallel to the road, but allow them to turn at the next intersection with other phases moving at the same time.

Right-of-way constraints are the primary concern for innovative intersections, which is why they have been applied in locations under development. Constructing an innovative intersection at an undeveloped site also helps establish access management principles before owners develop their property and set access points. Some treatments (e.g., median U-turn design) may be applied to urbanized arterials if the median is already present. Acquiring the right-of-way to widen the intersection could be a substantial task in fully developed areas.

In addition to right-of-way concerns, driver and pedestrian education is also critical; the new intersection signal operations may be unfamiliar to most road users. Public announcements, brochures and flyers, public meetings, and videos containing images of simulated operations (or of existing intersections at other locations) are all useful educational pieces. The Alamo Regional Mobility Authority has used these techniques on its website4 to show the public how to navigate the new superstreet intersections on Loop 1604.

The local Texas Department of Transportation office is the responsible agency for most innovative intersections. These designs are often located on major state highways. Local governments may also wish to consider such treatments on new city roads.

Innovative intersection project timelines are often lengthy, due to the need for a detailed design, public education, and construction activities. The Loop 1604 superstreet plan4 consists of three projects, each with its own multiyear timeline to obtain environmental clearance, complete the design, and perform the construction while carrying traffic through the area.

Construction activities on a recently completed superstreet project on US 281 north of San Antonio, described as an interim solution,4 were completed in less than a year. However, that construction took place without building additional travel lanes, and the time frame does not include previous design tasks.

The environmental assessment for this corridor began in 2008, the superstreet concept was proposed in January 2009, and a public meeting to discuss early design concepts and operational benefits was conducted in June 2009, with a completion date in October 2010. The overall time frame of this project was approximately 30 months.

In contrast, a continuous-flow intersection project in Utah with fewer environmental assessment concerns took 18 months from the beginning of the environmental document process to the end of construction.5

Depending on adjacent land use and design needs, costs of innovative intersection projects can be higher due to the need for more right-of-way and more complicated construction. There are few completed projects for each type of intersection and location, but estimates from early installations are documented.

The recently opened continuous-flow intersection at Bangerter Highway and 3500 South in Salt Lake City, Utah, was reported to cost $7.5 million. The continuous-flow intersection at US 30 and Summit Drive in Fenton, Missouri, had a construction cost of $4.5 million.3

• The existing roadway and median width.
• Segment lengths and widths.
• The adjacent development and access needs.
• The available right-of-way.
• Intersection/interchange geometry data.
• Intersection/interchange spacing.
• Traffic volumes (including peak-hour volumes).
• Turn counts.

• Pedestrian and bicycle counts.
• The typical speeds on the intersection.
• The intersection’s crash history (type and frequency).
• The typical delay experienced.
• The available financial support for the project.
• The current and optimized signal timing plans.
• The intersection’s hardware capabilities.
• The system’s coordination capabilities.