Just take Exit 111 at Marvin Road, also known as the Hawks Prairie area.
The state’s first “diverging diamond” interchange opened there with six lanes of traffic in August. More signal timing and landscaping will be done before the job is complete in early 2021.
A second version will be advertised for construction bids this summer in Snoqualmie, the centerpiece of a $147 million overhaul at the treacherous Interstate 90 to Highway 18 junction.
Drivers from either side approach the Lacey overpass using the right three lanes, then curve to the left side while above the freeway, then return to land while curving back to the right side.
Confused? Here’s the key: Because you’re on the left side, it’s possible to make a gentle left turn to enter the freeway without stopping.
Overall, there are only 14 conflict points where crossing or merging vehicles can collide, instead of the 26 points in a typical interchange. There are only two traffic-signal phases instead of three, so more green time for cars.
In Lacey, that means drivers reach I-5 sooner, and spend fewer minutes clogging the vast retail boulevards.
“It really has increased flow and reduced the delay,” said Blake Knoblauch, executive director of the Lacey South Sound Chamber of Commerce. Drivers who previously waited through two or three light cycles now usually make it through on the first green light, he said.
Among other benefits, supporters hope the new design in Snoqualmie will help drain off the congestion that backs up toward westbound I-90 — creating a risk of rear-end freeway crashes.
“You’ve got to try and fix that as soon as you can. A DDI (diverging diamond interchange) is ideal in that situation,” said Gilbert Chlewicki, known as the father of the design. The word “diverging” means all drivers who approach must diverge, to the left or right, he explained.
About 150 have been built across the U.S., said Chlewicki, founder of Maryland-based Advanced Traffic Solutions.
Nationally, a study of 26 of the interchanges in 10 states found total crashes dropped by 37% and injury crashes declined 54%, compared to standard interchanges with similar traffic loads, according to North Carolina researchers. Sideswipes are more frequent, but there are fewer rear- and side-impact collisions, they found.
Washington state doesn’t have traffic-flow data yet, because COVID-19 has disrupted any comparisons. The state will need about five years to gather a meaningful trove of safety statistics.
“It’s a positive direction for WSDOT, [Washington State Department of Transportation] to be seeking out these solutions,” said Carl See, deputy director of the Washington State Transportation Commission.
The $31 million project in Lacey, along with $281 million in freeway expansions near Joint Base Lewis-McChord to the north, demonstrate how Washington state remains committed to absorbing vehicle trips, despite its rhetoric about slashing greenhouse-gas emissions and driving.
The Hawks Prairie interchange handles about 60,000 vehicles per day. On the Cascades side sits a broad retail district that includes Home Depot and Costco, then houses. The Puget Sound side is filled by townhomes, light industries and warehouse distribution centers nestled in the trees.
The design seemed odd to Melissa Julich, a clerk at the Nisqually Markets Tobacco Outlet, the closest shop to the freeway.
Julich said the first few times she commuted home, she slowed while driving the left side of the overpass, feeling a bit paranoid that if she ventured out, a car would blindside her.
“It was a little bit weird, having to go on the intersection without worrying about getting hit,” she said. That took adjustment, after hundreds of past trips where she had to wait mid-overpass for a green left-turn arrow to enter northbound I-5.
Going home, she still needs five minutes to nose her compact car from the shop entrance into Marvin Road, when some kind soul obliges. But from there, it’s an easy trip to the freeway. “I kind of like it, actually,” she said.
The new intersections can be awkward, said Aerion Hall, who was delivering packages to retail stores in a 24-foot-long box truck. During congestion, it’s easy to get caught in “no man’s land” where the truck hood or tail obstructs cross-traffic if cars ahead don’t make it all the way through the atypically long intersections. “If I stick out and get hit, it’s on me.”
“It is definitely strange and a bit scary,” Hall said. “A lot of the time, you get people not wanting to let you merge into the correct lane to get onto the freeway.”
Traffic was thick enough before noon last Wednesday that a line of vehicles, including a log-truck driver, couldn’t travel from Marvin Road to the overpass on the first green light because so many cars filled the far-left lane leading to the I-5 southbound ramp to Olympia.
Pedestrians cross the freeway on a path in the center of the overpass, with a concrete barrier on either side. Hand-activated signals and short trails aid walkers crossing the exit curves.
Bicycling conditions remain hostile. WSDOT marked a 4-foot-wide bike lane between three vehicle lanes and the barrier — which cyclists may avoid by entering the pedestrian path. Chlewicki admits bike safety is a problem that he’s raised with state transportation agencies.
Walking, bicycling and transit use are minimal at Marvin Road anyway, because of the parking lots and near-zero housing next to the junction.
The diverging diamond roadway was $40 million cheaper than an expanded traditional interchange because WSDOT could fit the design on the existing Marvin Road overpass, which needed only 11 feet of widening.
WSDOT built a related lane design in Silverdale back in 2007, at the junction of highways 3 and 303. That overpass functions like a “single point urban interchange” (SPUI), with all the left-turn lanes and through-lanes alternating using just a single, very long intersection. But in Silverdale, the through lanes never shift to the left side of the road deck as they do in Lacey.
The Tulalip Tribes this year built a SPUI over I-5, at 116th Street Northeast next to Marysville, and the state is considering newfangled designs in Clark County where I-5 passes Northeast 179th Street.
In Lacey, the official purpose of the diverging diamond is to keep up with traffic growth through 2025. That may require yet another project that’s been considered by the city of Lacey — a direct truck road from the freeway to the nearby warehouses.