Naperville looks at innovative "diamond interchange"

Naperville will join a select group of US cities if it opts to build a diverted diamond interchange to address traffic congestion at IL 59 and I-88. According to the Illinois Department of Transportation, this innovative design has only been used in Missouri, but is currently planned or under construction in several other locations.

How does the diverted diamond differ from a traditional interchange? While the ramp configuration is similar, traffic on the cross-street moves to the left side of the roadway in the segment between the signalized intersections at either end of the bridge.

By moving traffic to the left, vehicles can perform a free-flow left turn onto the interstate without the aid of a traffic signal. This design also allows traffic exiting the interstate to perform free-flow left turn movements onto the cross street.

Diverted diamonds lower traffic speeds between the interchange and the cross-street, and have proven to be safer than traditional interchanges as a result.

This traffic calming effect does not mean less efficiency, though. Traffic signals will operate with more “green time, allowing more traffic to move through the interchange without stopping. Up to 60 percent decreases in overall intersection delay have been measured.

This design is pedestrian/bicycle friendly. On-street bikers follow outside through lanes, off-street bikes cross with pedestrians, and pedestrians cross at signals. The crossing distances for pedestrians are reduced. Naperville’s diverted diamond interchange at IL 59 and I-88 would direct off-street bicyclists and pedestrians onto a protected median through the center of the interchange.

In addition to the interchange, this project involves reconstruction of 3 miles of IL Route 59 between Ferry Road and Aurora/New York Street. The design includes a 10-foot-wide multi-use trail within the right-of-way. For more information, contact IDOT at 217-782-7820 or www.dot.state.il.us.