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EDC 2 Virtual Spring Summits - 21st Century Solutions

Q & A from Virtual Summits - Intersection & Interchange Geometrics

Q: What types of public outreach and driver educational assistance have been developed for states and local governments to use to help make it easier for drivers to understand these intersections?

A: Varieties of educational materials have already been developed by several agencies. As part of EDC, the FHWA intends to develop additional materials that agencies may use to help educate drivers and stakeholders.

Q: Back in 2000, the FHWA helped advance roundabout awareness and design by producing an informational report on roundabouts. Does FHWA intend to produce similar documents for U-turn treatments, displaced left turn intersections and the diverging diamond interchange?

A: In April 2010, FHWA published the Alternative Intersections/Interchanges Informational Report (available at: http://www.fhwa.dot.gov/publications/research/safety/09060/) which is the most comprehensive resource at this time. As part of EDC, FHWA will be producing updated, stand-alone guides for the RCUT, MUT, DLT and DDI. These updated guides will summarize recent studies and practices from recent projects to create more practitioner-oriented guidance and design materials. This guidance will include documentation on alternatives analysis and applicability criteria on how to decide what design treatment is the best fit for a location.

Q: The second edition of the FHWA publication Roundabouts: An Informational Guide (published as NCHRP Report 672) included a crash prediction methodology for roundabouts. Are efforts underway to develop methodologies for the other intersection alternatives highlighted in this EDC initiative?

A: Various efforts are underway to collect additional information on the safety performance of these intersections. It is envisioned that as more data becomes available, future efforts to develop crash prediction methodologies will be possible.

Q: Are these intersection designs safer for pedestrians?

A: As at all intersections, turning vehicles and the speed at which they travel pose the greatest threat to pedestrians. Also, oftentimes at intersections the motorist's attention is focused on other motorists and not on pedestrians. These intersection designs offer advantages with regard to reducing or spreading conflicts points, including certain pedestrian-vehicle conflict points. Good pedestrian design practices should be applied at these, and all, intersections to address the needs for all users. Each of these designs has advantages, and potential disadvantages, when striking a proper balance of user mobility and safety. In most cases, designers will have many options available to consider regarding pedestrian pathway travel and the potential safety and mobility benefits and trade-offs.

Q: Are these intersection designs compatible with the Americans with Disabilities Act (ADA)?

A: The Americans with Disabilities Act (ADA) provides minimum design standards to be applied to all public environments, including the public right-of-way. The Americans with Disabilities Act Accessibility Guidelines (ADAAG) are the foundation for designing all pedestrian environments and requirements for public rights-of-way are currently in draft form. For these intersection designs, and all intersections, it is critical for transportation providers to understand the principles and details for accessible design and apply good engineering judgment in design to make the features/facility accessible. For multi-lane roundabouts and multi-lane channelized right-turn lanes, activated accessible pedestrian signals (APS) should be provided for each segment of each crosswalk, including the splitter island. For further information, see NCHRP Web-only Document 117A, Accessible Pedestrian Signals: A Guide to Best Practice. Pedestrian accessibility enhancements not only benefit people with disabilities; they benefit all pedestrians.

Q: How do these alternative intersection geometrics handle bicycle traffic?

A: With these intersection types there are varying degrees of experience with regard to the options for providing bicycle facilities. Much depends on whether there are marked bike lanes, vs. off-road path, vs. in-lane. Perhaps the most challenging condition is experienced at the displaced left-turn (DLT) intersection, but bicycle provisions at the other intersection types are more straightforward. The great thing about any of these types of intersections is that they encourage thoughtful consideration of pedestrian and bicyclist needs and frequently present great opportunities to do better than traditional solutions and not take these user group needs for granted.

Q: Can Federal safety funds be used to construct these alternative intersections?

A: All transportation projects funded through a variety of Federal and state sources should include an explicit consideration of safety. To most effectively and efficiently apply the limited funds available through the Highway Safety Improvement Program (HSIP), criteria have been published and are available on FHWA's HSIP website: http://safety.fhwa.dot.gov/hsip/resources/fhwasa1102/flyr1_el.cfm The HSIP regulations (23 CFR 924) provide examples of the types of projects eligible for HSIP funding and is intended to be sufficiently flexible to allow states to program funds for those projects with the greatest potential to improve safety. The key criteria for eligibility are intended to prioritize safety projects identified through a data-driven process reflecting the state's Strategic Highway Safety Plan (SHSP).

Q: Roundabouts are proven in both low-speed urban and high-speed rural environments. What is the maximum recommended speed for a facility that roundabouts may be used?

A: There is no set recommendation along these lines. There is experience with roundabouts used along highways with posted speeds of 50MPH to 65MPH. The key to success for using roundabouts on high speed facilities is how to slow traffic on the approaches by using a combination of geometric elements and traffic control devices. In general, the higher the speed, the more advance treatment is necessary.

Q: Have any reports been published on the Benefit / Cost ratio associated with roundabout installations based on their proven ability to reduce collisions, severe collisions, and control delay?

A: Much depends on what is considered on both sides of the equation. In order to address this question on a national level, there is an NCHRP project in progress that will look at lifecycle cost analysis of intersections. I think we'll see some good answers come out of this project.

Q: At a restricted crossing U-turn (RCUT) intersection, doesn't it require a larger traffic gap to merge into a lane than to drive across the street?

A: Although the gap needed to turn (or merge) may be longer, the needed gaps are taken one at a time rather than needing gaps in all lanes to cross the major street or turn left.

Q: At the RCUT, aren't new conflict areas being creating where making the U-turns?

A: The overall conflict points are spread out at the RCUT and the driver decision points are better distributed. By reducing and spreading the conflict points and types, a net safety benefit results overall.

Q: What are the speeds of highways the RCUT has been used on (45MPH, 55MPH, 65MPH)?

A: The RCUT (or J-turn) has been successfully used on rural arterials of high speeds (55-65 MPH).

Q: For the U-Turn Intersections, are there established design criteria and applicability guidelines?

A: There is information on applicability criteria and geometric design parameters in the FHWA Alternative Intersections/Interchanges Informational Report which is still the most comprehensive resource at this time (http://www.fhwa.dot.gov/publications/research/safety/09060/). Additional efforts are currently in progress that will create more practitioner-oriented guides and materials.

Q. Can trucks make the U-turns?

A. Yes. The U-turn intersections should be designed to accommodate the turning needs of an appropriate design vehicle. The combination of geometric dimensions of the median, the travel lanes and the shoulder widths determine the design parameters and possible need for turning bays, or loons, to accommodate large trucks making the turns.

Q: At the U-turn intersections don't some drivers need to travel farther out of their way?

A: To make some movements drivers will need to travel away from the main intersection, make a U-turn and then return back creating some additional travel. However, the overall time savings to make the desired movements can be significantly less because of improved operations at the intersection. Also, safety is improved by reducing and spreading the traffic conflict points.

Q: Is it possible to utilize the U-turn intersection concept on roadways that do not have a wide median?

A: The ThrU-turn concept is an example of applying this scenario without a median. In the ThrU-turn geometric, the U-turn maneuvers are concentrated at downstream intersection areas.

Q: How is a diverging diamond interchange (DDI) safer than other interchange types?

A: The DDI has fewer conflict points at which vehicles can potentially collide. In particular, there is a great safety advantage by reducing crash points between vehicles turning left onto the freeway entrance ramp and the opposing arterial roadway traffic. The geometrics and traffic channelization at a DDI also reduces the risk of wrong-way freeway entry at the ramps.

Q: What are the operational benefits of a DDI?

A: By eliminating the conflict of left-turning vehicles entering the freeway entrance ramp against oncoming traffic, there is a reduction in the number of traffic signal phases A DDI reduces congestion by allowing more efficiency in accommodating turns when entering and exiting the freeway.

Q: Is a DDI cost effective?

A: Yes. On interchange improvement projects, a DDI can often be constructed using the existing bridge structure and within the existing right-of-way. Because many of the existing interchange features remain intact, the DDI is often constructed in less time than it would take to construct a new interchange and with less impact to motorists. When constructing a new interchange, a DDI usually requires fewer lanes and therefore a less expensive bridge structure than other interchange types.

Q: Is motorist confusion an issue for the DDI?

A: When looking at an aerial photo of a DDI, it might look confusing or a challenge to drive through. But in reality, a DDI has pavement markings and traffic signs and signals very similar to other intersections. When driving a DDI, motorists proceed through a traffic signal at the approach to the interchange, and simply follow their lane to the opposite side of the roadway. While on the left side, motorists wanting to access the freeway may make the turn left onto the on-ramp without having to stop for additional traffic signals or wait for oncoming traffic to pass. Through traffic proceeds past a second traffic signal and follows their lane back to the right side of the road. Signs, pavement markings and signals direct motorists to where they need to go. We strongly encourage you to visit a DDI and drive it for yourself. You may be very surprised at how intuitive it is from an "on-the-ground" perspective.

Q: Are there any changes proposed to the MUTCD regarding signing for a DDI?

A: Special signing is not needed for the DDI, and any unusual signs could potentially cause driver confusion rather than mitigate it. There have been some requests for experimental use of warning signs that attempt to diagram the lane cross-overs but these requests have been rejected by FHWA and experimentation for special signing deemed unnecessary at this time.

Q: Would the DDI create a more hazardous condition when the power goes out as compared to a traditional diamond interchange?

A: In the event of a total power failure, treating a 2-phase signal operation as an All-Way Stop is simpler than what would happen at a 3-phase intersection in a traditional diamond interchange.

Q: Can a DDI be snow plowed?

A: Yes. Snow can be plowed/removed at a DDI. Several states that experience heavy winter snows have successfully maintained a DDI during winter weather.

Q: What parameters define the minimum distance between intersections in a DDI?

A: The distance between the two crossovers is largely dependent upon the location and spacing of the ramps. The space needed for vehicular storage between the crossovers must also be considered. If there are no physical constraints, designers have flexibility in the placement of the crossovers. If greater length is needed than the distance between the ramp termini provides, the crossovers may even be located farther apart than the ramp spacing if the ramp entrances and exits can be configured to merge or diverge with the cross route by extending or shortening them. A well-designed crossover requires consideration of several horizontal geometric elements including: the crossing angle, the tangent length approaching and following the crossover and; the curve radii approaching and following the crossover.

Q: What is the minimum spacing to intersections adjacent to the DDI?

A: As with any interchange type, it is desirable to have sufficient intersection spacing to avoid operational problems. However, special consideration must be given in evaluating a DDI when there are adjacent signalized intersections since the DDI operates with two-phase signals with only one direction of travel on the crossroad allowed through the interchange at a time. When an adjacent signalized intersection is too close to the DDI, it greatly limits the ability to coordinate both directions of travel along the crossroad. The adjacent signals can result in traffic queuing into the crossover intersections. It does not appear that closely spaced right-in/right-out access pose any greater challenges for DDIs compared to other interchange types.

Q: Are there any examples of a 2-lane rural DDI?

A: At present we are not aware of a 2-lane rural DDI. For an interchange at a 2-lane rural highway, roundabouts might be a better choice than a 2-lane DDI.

Q: Is there a comparison between a DDI and using roundabouts at a diamond interchange?

A: When evaluating and comparing interchange alternatives, the use of roundabouts at the diamond interchange is generally a viable alternative that should also be considered.

Q: What modeling software should be used for these alternative designs?

A: Selecting an appropriate traffic analysis tool depends greatly on the needs for the analysis elements of the intersection/interchange. There are easy to use planning level tools such as the FHWA sponsored CAP-X tool that can be used to screen many of these designs based on the volume demands. For roundabouts, the Highway Capacity Manual offers a relatively simple and well accepted deterministic model for operations. In some instances, microsimulation tools may be needed. There are numerous variations among microsimulation tools in terms of their strengths and weaknesses. Like any tool, it must be used properly and is only as good as the inputs. Future traffic projections are a very important input element in any traffic analysis. It is suggested that when evaluating complex conditions with high volumes, a sensitivity analysis evaluating how changes in the traffic projections affect the results of the operational analysis should be performed. A sensitivity analysis helps demonstrate if the proposed alternative functions under a number of traffic conditions and could satisfy a variety of future traffic conditions.

EDC 2 Virtual Spring Summits - 21st Century Solutions


Page last modified on August 12, 2013.
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