What's a diverging diamond interchange? I-78 at Route 100 redesign will take drivers on ‘wrong side' of road

Motorists using or passing the Route 100 interchange with Interstate 78 likely have had a white-knuckle experience or two over the years.

The cloverleaf interchange in Upper Macungie Township was built in the 1950s, and with the industrial buildings that have gone up in recent decades along with the township’s population growth, using the interchange has become a daily dance between cars and tractor-trailers.

With the improvements slated for I-78 in western Lehigh County, PennDOT plans to convert the Route 100 interchange into a diverging diamond, which means there will be one entry and exit point for each direction of traffic instead of two.

“The way it's set up, it works well with the heavier traffic movements that are exhibited at the interchange," explained Joshua Golomb, an engineer with RK&K, which is working on the project. "It eliminates the traffic signals for that traffic, so they have free flow.”

PennDOT presented plans for future improvements on I-78 from Route 100 to the Berks County line Wednesday evening at Fogelsville Elementary. Other improvements include a new interchange with Adams Road in Upper Macungie and an additional lane in each direction to Route 863 in Weisenberg Township. A third lane will also be built eastbound approaching Route 863.

The project is currently in preliminary design through 2027 and then will enter final design. Portions of the project, including the new interchange with Adams Road, will begin construction in early 2030.

The remainder of the project will be completed in phases over the next five to seven years. The I-78 Reconstruction Project is fully funded through a combination of state and federal funding.

How a diverging diamond works

According to PennDOT, a diverging diamond interchange “enhances and simplifies the operation of a diamond-style interchange by moving the crossroad traffic to the opposite side at the on- and off-ramps to the interchange." Essentially, motorists on the crossroad travel on the left side of the road through the interchange.

This configuration creates fewer conflict points for motorists and allows for easier left turns onto the main highway, unlike a standard diamond interchange.

It also avoids traffic “weaving” created by cloverleaf interchanges like the one currently in place at I-78 and Route 100, where vehicles entering I-78 must cross paths with those exiting in a short amount of space.

The new design will have Route 100 northbound traffic to I-78 east exit right at its current point. However, traffic continuing to I-78 west will cross over to the left side of the road (with a traffic signal controlling flow). After crossing the I-78 underpass, traffic to I-78 west will merge to the left, mixing with traffic from Route 100 south for a single entry ramp onto I-78 west. Traffic continuing northbound on Route 100 will then cross back over to the right side of the road.

The pattern will be similar for Route 100 southbound.

It will be the first DDI constructed in the Lehigh Valley, and Golomb said many are being built across the state. Presently, the nearest completed one to the Valley is at Route 222 and Route 322 near Ephrata, Lancaster County.

Down the road, the design is being eyed for the Route 22 interchange at MacArthur Road as part of the future Route 22 widening project, and for the Route 33 and William Penn Highway interchange in Bethlehem Township.

Golomb said the design has been improved in recent years, and drivers should quickly adjust to how they work.

“There is a warm-up period," Golomb said. "It's very straightforward, because when you go through it, you're approaching a two-way intersection, so you don't really feel like you're doing anything different.”

The estimated cost for the interchange improvements is $120 million-$130 million.

PennDOT has posted the plans online, and is taking public comment and questions through June 5.

Morning Call reporter Evan Jones can be reached at ejones@mcall.com.

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