In KIRO Radio’s Chokepoints series, we take a closer look at the traffic issues plaguing commuters in the Puget Sound region.

Anyone who has driven I-5 south into Seattle in the afternoon knows this one. The dreaded Ship Canal shutdown, the backup that often extends into Northgate.

There are a lot of ideas on why the Ship Canal Bridge backs up every day. But KIRO Radio listener Ed believes that allowing drivers to access Highway 520 from the 45th Street on-ramp is the reason why it grinds to a halt.

Ed blames the multi-lane weave drivers need to do to get all the way across from the right to the left side of the bridge for the backup. Ed thinks the state should prevent drivers from doing this.

Travis Phelps at the Washington State Department of Transportation says it’s not that easy.

“Just simply saying you can’t change lanes here is not necessarily solve your problem,” he says. “You’d be, kind of, shifting that weave point somewhere else throughout the transportation system.”

Phelps says restricting access to 520 from 45th would create chokepoints in other places. Those drivers would have to get on I-5 somewhere else. They would crowd an already crowded U-District as they try to get down to Montlake to access 520.

He says there are other reasons beside the weave that contribute to the Ship Canal shutdown.

“We’ve also got the grade of the Ship Canal Bridge and you also have a tremendous view off of that bridge that a lot of people take into account,” says Phelps.

You also have the sheer volume of drivers that are headed into downtown.

So Ed, the weave is going to stay, but it’s not the only culprit for the Ship Canal shutdown.