I5 and 205 Bridges Question. (Portland, Sandy: live in, airport, moving)

[eric351982]

I have a question that continues to baffle me. I'm a reasonably intelligent guy but I can't figure this out for the life of me.

The traffic going north on I5 and 205 is absolutely jammed, barely crawling along every day. Why is it that when you get to the bridge you immediately speed up to 45 mph by the middle of the bridge and up to 60 mph by the end of the bridge? Why? It's like there's some sort of portal right at the start of the bridge that ends the traffic. There is no traffic north of the bridge in Vancouver.

I was driving on Marine the other day and saw this. 205 was stopped and people a quarter of a mile onto the bridge were going 55 mph.

I'm terribly curious.

[Dom1964]

very simple... Lots of people getting ON the bridge in NE Portland, causing gridlock... and lots of people getting OFF the bridge as soon as they cross into Vancouver, opening things up.

[urbanlife78]

Hwy 14 helps disperse the traffic going over the bridges. Basically once on the other side, there is multiple ways people will go, but to cross the river there is only two.

[AndyAMG]

For I-5 its 3 on ramps in a row. Delta park/marine/jantzen beach are all jamming cars onto I-5 in one mile. Then the same on 205 with 84/Sandy and Airport Way.

Cars jam up in the right lane so then other cars keep moving over until all 3 lanes are going slow. Then of course there is the semis. Semis, screw everything up for everyone trying to get some where because they can accelerate and leave a ton of space between them and the car in front. They can't climb the gradient with ease so they do 35-40 in the right two lanes because inevitably they want to pass each other, then you have timid grandma passing them in the left most lane at 40 because shes afraid of the wall on her left and the giant semi on the right bumping and swerving about.

I-205 is a bit better because trucks have the decency to stay in the 2 right lanes. On I-5 they do the same but with only 3 lanes lanes they only leave one open.

After the trucks and grandma drivers are over the hump on I-5 and it's not so narrow they resume to normal driving and.... no more cars are introduced in large quantities so I-5 starts flowing again. But it starts slowing again at SR500.

On I-205 its the same thing but I think its because grandma drivers just stay with the trucks and faster drivers are able to escape in the left two lanes. If you notice the opening up doesn't last long after SR 14. It all backs up again at mill plain then again at SR 500 then crawls all the way to I-5.

But there is also its just human psychology.


https://www.youtube.com/watch?v=Suugn-p5C1M

https://www.youtube.com/watch?v=goVjVVaLe10

Cliff notes.... too many cars for too little road in too short of time.... lol

Of course you could live in china

https://www.youtube.com/watch?v=Y8bfNplEmfo
https://www.youtube.com/watch?v=jI83lIw99u0

[bler144]

Agree with Andy, though I'd say it's not just those last 3 on-ramps, it's all the exchanges between 405 and Jantzen beach (8 in 6 miles), though the 3 at the end are definitely the logjam that breaks the camel's back.

And not just the cars merging on, but the cars merging right to get off.