Bowtie

A Bowtie road junction.

The Bowtie is an alternative to the Michigan left format that uses roundabouts on the cross street. Left turns are prohibited at the main intersection, which therefore requires only a two-phase signal. Vehicles yield upon entry to the roundabout, but if the roundabout has only two entrances, the entry from the main intersection does not have to yield. The roundabout diameter, including the center island and circulating roadway, varies from 90 feet to 300 feet depending on the speed of traffic on the approaches, the volume of traffic served, the number of approaches, and the design vehicle.

This configuration would reduce delay for the arterial street, increase capacity, and reduce the number of stops required. The primary problem with such a configuration is driver disregard for the left turn prohibition at the main intersection.

As of yet no agency has designed a complete bowtie road junction.

A dumbbell interchange

A grade-separated version of this junction is a dumbbell interchange, where 2 raindrop roundabouts are fused together. This is basically a squashed version of a roundabout with a by-pass (usually overpass) for the main road. The advantage is that the narrow center of the two roundabouts need only a small footprint under the overpass. A disadvantage is that traffic needing to go left (in countries where is driven on the right) must make a series of curves that can be uncomfortable.

References

• Unconventional Left-Turn Alternatives for Urban and Suburban Arterials An Update (pdf)

• Eliminating Left-Turns at a Signalized Intersection With The Use of Nearby Roundabouts (pdf)

• Guidance on the Safe Implementation of Unconventional Arterial Designs (pdf)

• FHWA-HRT-04-091: Signalized Intersections: Informational Guide

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