Operational Aspects of the Michigan Urban Diamond Interchange

As the Interstate highway system was constructed, a large number of freeway interchanges were designed and constructed. The design standards for these freeway interchanges, however, were not derived from an indepth analysis of past experience, because there was very little experience from which the design engineer could draw. Instead, most designs were replications and modifications of existing freeway interchanges. Thus, many of these early interchanges precede the Interstate system and Interstate standards. The simplest and perhaps most common interchange used is the urban diamond. Unfortunately, this configuration has relatively low capacity because all the turning movements occur at the intersections and left-turning vehicles have to yield to oncoming traffic. The Michigan Department of Transportation (MDOT), borrowing from its indirect left-turn strategy implemented for most at-grade urban boulevards, modified the traditional urban diamond in an effort to increase the design’s capacity. This modified diamond-interchange configuration is referred to as the Michigan urban diamond interchange (MUDI). Michigan State University and MDOT are jointly evaluating the merits of a MUDI geometric configuration. Computer modeling with TRAF-NETSIM was used to operationally compare the diamond to the MUDI. The TRAF-NETSIM simulations run on each type of interchange give valuable insight into which design is better under different volume and percent turning conditions. Operationally, the MUDI was superior to the diamond interchange in most cases. Additionally, the MUDI configuration does not transfer delay to downstream nodes, whereas the diamond interchange with frontage roads appears to affect the operation of these nodes.