Incorporating Alternative Intersections and Interchanges Into Highway Capacity Software (HCS 2010)
By: Bill Sampson, P.E.
The use of alternative intersection and interchange designs in transportation engineering is continually becoming more popular in the United States – approximately fifty exist today, with many more currently in the planning and development stages. As a result, practicing civil engineers that specialize in transportation planning and traffic operations must be able to accurately quantify their performance (i.e., capacity and level of service) to make informed design and operations decisions.
The McTrans Center within the University of Florida Transportation Institute (UFTI) is collaborating with several researchers and investigators as part of an ongoing Federal Highway Administration (FHWA) project to define methods for integrating new procedures and methodologies for these alternative intersection and interchange designs into the Highway Capacity Manual (HCM 2010). In particular, the FHWA is interested in four at-grade designs: the Median U-Turn (MUT), Restricted Crossing U-Turn (RCUT), Displaced Left-Turn (DLT) and Diverging Diamond Interchange (DDI).
While these alternative designs may initially require some time to establish familiarity and consistently meet driver expectations, many experts in the field believe the investment is worth considering. Conventional signalized intersections that experience excessive levels of congestion and delay cannot always be solved by simply adding extra lanes or expanding the right-of-way – in many cases, the necessary funding and land space simply do not exist. These four designs are examples of innovative solutions conceived by transportation engineers that maximize capacity and level of service while minimizing land use and construction costs.
The first step in the process was collecting as much information on the designs as possible from a number of sources; this included observing microscopic simulation videos as well as evaluating research publications, operational reports, and design guidelines for all four designs in order to become familiar with all of the unique intricacies and nuances within the each of the designs. Secondly, a series of origin-destination tables were developed in order to equate peak-hour turning movement counts of conventional intersections to those of alternative intersections, as well as to account for additional movements previously undefined (U-turns, for example). Then, the designs were incorporated into the Streets module of the HCS 2010 by developing a number of shortcuts in order to model the unique features of these designs.
Many components of the expanded procedures and methodologies are still forthcoming; “gaps” or limitations with the current HCM 2010 have been identified and are being addressed using statistics-based analytical techniques based on recorded video feeds stationed at several existing alternative intersections and interchanges located throughout the United States. One of the more significant concepts being considered is the use of travel time as a parameter for measuring level of service, potentially replacing control delay used for conventional signalized intersections. This would account the additional distances required for some movements in these designs. Right-turn on red (RTOR), yield-controlled right turns, and right turns not under signal control (free rights) are also being analyzed. Other familiar concepts that must be further evaluated to account for new geometric features within these alternative designs include lane utilization factor, adjusted saturation flow rate, upstream filtering adjustment factor, free-flow speed, deceleration time, and queue storage ratio to name a few.
The ability to model alternative intersections and interchanges within HCS 2010 provides the tool for the research process to evaluate each of these design features. Once the procedures are finalized within the HCM 2010, the HCS 2010 will be modified to provide this capability to end users within a more intuitive interface application. Just as roundabouts were once considered inventive and groundbreaking when they were introduced in the United States in the 1990s, alternative intersections and interchanges are considered by many in the field to be the wave of the future in transportation engineering.