Operational Aspects of Michigan Design for Divided Highways

Abstract

Multilane trunk-line (state routes other than freeways) highways in the state of Michigan typically have one of two cross sections: five-lane or boulevard. The cross section of a five-lane facility consists of four through lanes and a continuous two-way center left-turn lane. The alternative to the center left-turn lane design is a divided highway with a median. The median widths may vary from as little as 1.2 m (4 ft) to more than 30 m (100 ft). Since left turns at intersections have a major effect on limiting the capacity of the intersection, Michigan uses directional U-turn crossovers to provide for an “indirect left-turn” maneuver at signalized intersections. The alternative to this boulevard strategy is to provide for direct left turns with a left-turn phase at the intersection. It was impossible to find appropriate locations in the field to evaluate the sensitivities of these designs and operations with various traffic volumes and left-turn volumes. Thus, computer modeling, via TRAF-NETSIM, was used to evaluate these designs operationally. The boulevard designs that used indirect left-turning strategies and signalized crossovers were superior, in most cases, to all other designs considered. The boulevard designs that used direct left-turning strategies had proportionally higher amounts of delay than all other designs considered, and their operation tended to fail as the percentage of left turns and traffic volume increased. In addition, with low percentages of left turns, the operation of a boulevard with an indirect left-turn strategy was similar for both signalized and stop-controlled crossovers.