Diverging Diamond Intersections (DDI) are a recent innovation in intersection treatment to speed up the flow of traffic and reduce conflicts where two major highways intersect. DDIs are a very effective method to reduce crashes and smooth the flow of motor vehicle traffic.
Because of the geometry of a DDI, riding and walking traffic is difficult to accommodate and has raised controversy about use of this design.
Photo Above: Aerial photo of I-44 / Kansas Expressway Diverging Diamond Interchange in Springfield, Missouri. First of its kind in the U.S. Photo from Missouri Department of Transportation. Taken from http://www.divergingdiamond.com.
There are many variations of DDI that have been built in the last few years but all create problems for the reasonable flow of non-motorized traffic. The photo above shows (from lower right) a sidewalk with a crossing over an unsignalized right turn slip lane, a crossing to a center median walkway with barriers on either side, and then similar crossings on the far end. Consider the dangerous situations a person in an electric assist device or a vision-impaired pedestrian would face attempting to use this intersection. Consider as well a family with small children in a stroller.
The Federal Highway Administration (FHWA) policy statement on accommodating bicyclists and pedestrians contains the following language:
“4. The design and development of the transportation infrastructure shall improve conditions for bicycling and walking through the following additional steps:
- planning projects for the long-term. Transportation facilities are long-term investments that remain in place for many years. The design and construction of new facilities that meet the criteria in item 1) above should anticipate likely future demand for bicycling and walking facilities and not preclude the provision of future improvements. For example, a bridge that is likely to remain in place for 50 years, might be built with sufficient width for safe bicycle and pedestrian use in anticipation that facilities will be available at either end of the bridge even if that is not currently the case
- addressing the need for bicyclists and pedestrians to cross corridors as well as travel along them. Even where bicyclists and pedestrians may not commonly use a particular travel corridor that is being improved or constructed, they will likely need to be able to cross that corridor safely and conveniently. Therefore, the design of intersections and interchanges shall accommodate bicyclists and pedestrians in a manner that is safe, accessible and convenient. (emphasis added)
- getting exceptions approved at a senior level. Exceptions for the non-inclusion of bikeways and walkways shall be approved by a senior manager and be documented with supporting data that indicates the basis for the decision.
- designing facilities to the best currently available standards and guidelines. The design of facilities for bicyclists and pedestrians should follow design guidelines and standards that are commonly used, such as the AASHTO Guide for the Development of Bicycle Facilities, AASHTO’s A Policy on Geometric Design of Highways and Streets, and the ITE Recommended Practice “Design and Safety of Pedestrian Facilities“.
The Utah DOT has a document that talks about DDI and shows the bicyclist and pedestrian accommodations that can be done. Exhibits from that document that define bicyclist and pedestrian design options are shown below.
In the design section is a table of Lessons Learned. The first item on the list is pedestrian fencing on both sides of the DDI.
IH-35 at FM 1431
The Texas Department of Transportation (TxDOT) is currently completing construction on a DDI at IH-35 and FM 1431 in Round Rock, TX, just north of Austin, TX. It became operational on November 19, 2015 and is the second DDI to become operational in Texas. This DDI is still under construction and the limited sidewalks are blocked off and there are no warning signs associated with the pedestrian crossings.
A diagram taken from TxDOT presentation on the DDI at FM 1431 shows no accommodation for cyclists or pedestrians.
Pedestrians attempting to cross from north to south in the shopping complexes on the east side of IH-35 are not accommodated with the design of the DDI on 1431. The west side of IH-35 is currently not as developed as is the east side but it appears that there is no accommodation for pedestrians crossing on that side either. The sidewalk does not allow for crossing of the main travel lanes. Not all pedestrians are able bodied. People using an electric assist device or pedestrians with vision impairment are poorly served with the sidewalk as it is currently configured.
Moving from east to west the only sidewalk is on the right side of the bridge (north) and is narrow, high off the travel surface, has no barrier between the sidewalk and traffic, and is facing traffic coming from the west. It appears that this sidewalk may not meet American with Disabilities Act (ADA) standards for width or protection. It certainly does not meet the criteria that TxDOT posted on their website about this project.
“Other benefits of a DDI include:
- Motorists are able to bypass the intersection without stopping at a traffic signal
- Improved travel time because of additional “green time” at traffic signals allow more vehicles to pass through the intersection
- Additional sidewalks increase safety and better accommodate bicyclists and pedestrians” (emphasis added)
This crossing is similar to the “perimeter ped crossing” shown in exhibit 2.8 ddi pedestrian crossing from the Utah DOT document.
FM 1431 is a critical east/west crossing of IH-35 for cyclists. It is the only crossing of IH-35 between Old Settler’s Blvd and Westinghouse Rd a distance of 3.2 miles. It allows cyclists coming from the residential and medical areas to the east access to the Texas Hill County and Ronald Regan Blvd, a major cycling route. It also allows access to the bicycle facilities associated with US 183A that link with Cedar Park and Leander.
As currently configured cyclists traveling east on FM1431 will be riding on the shoulder in 60 MPH traffic. Approaching the DDI the shoulder disappears and cyclists are required to occupy the rightmost lane in 45 MPH traffic. No shoulder or bikeway is provided by the design of the DDI from the Right Turn Only lane leading to Austin, across the DDI to the shoulder that disappears for the Right Turn Only lane leading into the shopping complex on the east side of IH-35. While it is legal to occupy a lane at 45 MPH and will be done by advanced cyclists especially those in a group, for an individual cyclist or one who is less advanced occupying the lane at 45 MPH may not be a viable alternative.
Therefore, the design of the DDI has removed the west to east link provided by 1431 for most of the cyclists who currently use this crossing.
As currently configured cyclists traveling west on FM1431 have the option of occupying the right most lane that leads across the DDI or taking the sidewalk. Occupying the lane in 45 MPH traffic is legal and will be done by advanced cyclists especially those in a group. For an individual cyclist or one who is less advanced occupying the lane at 45 MPH may not be a viable alternative.
Taking the sidewalk as it is currently configured requires traveling facing traffic on a raised narrow sidewalk with no barriers between the cyclist and traffic. A mistake on the part of the cyclist will mean a fall into oncoming traffic. TxDOT has indicated that cyclists using the sidewalk will be required to dismount and walk.
The access roads on both sides of IH-35 have traditionally been one of the ways that cyclists move north/south along the corridor. The current configuration of the DDI at FM 1431 has significantly increased the danger faced by a cyclist using this former route.
Previously the access road met FM 1431 at a traffic light. Cyclists would stop at the light and proceed across FM1431 on the green signal. Motor vehicles or cyclists turning right or left on 1431 would move to the proper lane and proceed with the green signal.
Cyclists wanting to turn onto the access roads to travel north or south from FM1431 would cross in the rightmost lane to the light on the far side, stop and wait for the light to change then proceed directly down onto the access road using the rightmost lane and shoulder.
Now, cyclists going straight will have to merge across two lanes of high speed traffic to be able to continue on the access road that leads under the FM 1431 overpass. On the other side the cyclist will have to merge back across two lanes of high speed traffic to right to the right as required by law.
Cyclists turning right are accommodated by the current design. However, cyclists turning left must merge across two lanes of potentially high-speed traffic to enter the Left Turn Only lanes.
Cyclists wanting to turn onto the access roads from FM 1431 are required to occupy the rightmost left turn lane that crosses the bridge and then merge with turning traffic coming from the opposite direction on FM 1431.
TxDOT has removed relatively safe accommodations for cyclists and pedestrians and has replaced the removed accommodations with limited, un-safe accommodations for pedestrians and no accommodations for cyclists.
The design of the roadways around this DDI has made use of the common way more dangerous for cyclists and pedestrians effectively banning cyclists, pedestrians and persons with handicaps from access to the roadway.
Caveat: Construction is not complete and changes still to be made in the intersection could change the conclusions stated in this document. Currently there is no signage warning motorists of pedestrian crossings. Some pedestrian signal heads are in place at crossings and more are likely planned. A barrier between the sidewalk and motor vehicle traffic may be planned. However, these changes would not change the basic conclusion that the safety of pedestrians, persons with disabilities and cyclists has been significantly compromised with the DDI.
If you wish to see what traffic flow in this DDI looks like here is a short video taken on Friday afternoon December 19, 2015 at 2:00 pm. Traffic may have been heavier than usual due to the Christmas Holiday.
This document was prepared by W. Preston Tyree of cycleSMARTER is intended to open dialogue about the need to accommodate people who ride, people who walk and people with disabilities in all of our public roadways.