1

Actually it's a tradeoff. It's more efficient if there's a lot of traffic moving onto or off of the highway at that intersection. If there isn't very much, it's less efficient.

I wonder if it was an experiment, inreal world conditions, to see whether it was a good tradeoff.

2

You're probably right, it's likely an experiment.  I'd be very surprised if the tradeoff would be much worth it, though.  You're trading an awful lot of dead weight on that bridge in the median on that bridge to save some relatively cheap traffic signaling.  You'd have to have an awful lot of traffic backing up at the signals to be worth the expense.

It is very interesting, nonetheless.  Springfield isn't far from here; I might have to go look at it.

3 The official term for that which enables on to follow up on it is "diverging diamond interchange". I particularly recommend the video wikipedia links to which I found very helpful for visualizing it: http://www.modot.org/stlouis/major_projects/OverheadPlanView_000.mpg

I drive through this fun little number most weekdays. I don't mind it too much but many of my family members will drive an extra 5 minutes if it means they avoid that.

4

Jeremy, I don't like rotaries. I seriously got my fill of them while living in Massachusetts because I lived near this one.

What isn't apparent there is that an 8 lane highway dumps most of its traffic through that rotary during rush hour. Why they don't rack up several deaths a day in that rotary I'll be damned if I know. I was always terrified driving through it.

5 We have a variant on that theme here in Everett. 41st St. (see if I did that right)

If not:

http://maps.google.com/maps?q=Everett+Wa&hl=en&ll=47.964597,-122.19919&spn=0.006501,0.018572&sll=42.5058,-83.755474&sspn=0.028632,0.074286&vpsrc=6&t=h&z=17

http://g.co/maps/av45

Jeremy's reminded me of this one in Gig Harbor I had to deal with twice a month when I serviced Office Depot stores.

http://maps.google.com/maps?hl=en&ll=47.359351,-122.616091&spn=0.006577,0.018572&sll=42.5058,-83.755474&sspn=0.028632,0.074286&vpsrc=6&t=h&z=17

Gah, sorry about the hash....  I'll figure out this link inset tool yet....

6 Anyway, the idea of the 41st st. interchange was to merge the two intersections into one on top of the bridge, and make it take up less space, although the overpass is much wider than it was before.

7 I see a huge, possibly insurmountable problem with that bridge.

The human interface.

This traffic pattern goes directly against drivers' instincts and ingrained habits.  If I was approaching this bridge blind, having never seen it before, I would certainly slow, holding up traffic, while trying to verify that Yes Virginia, you're supposed to drive on the wrong side of the road and then cross back at a point that you cannot see from ground level, especially behind other traffic.  Want to make it worse?  Try coming across it at night, in the rain, when the painted lines tend to be the hardest to see.

Any benefits gained by removing the left turns from having to cross traffic could easily be negated (and then some) by the confusion and timidity of newbies.  In addition, something just feels dangerous to me to be teaching drivers to ignore their otherwise-legitimate instincts for staying on the right side of the road.

Don't get me started on traffic circles.  Those abominations are deathtraps for non-locals, and insanely inefficient, especially when heavily loaded.  I've noticed a trend for ritzy, upscale suburbs to sometimes put them in, and it infuriates me.

I can't stand cloverleafs, either--you route slow-moving traffic onto a freeway a hundred yards before the freeway traffic has to exit onto its own cloverleaf.  That's insane.  The best type of interchange I've seen is what I've always heard called the mix-master (wikipedia says it's a "stack interchange"), which separates each flow vertically and limits all turns to no more than 90 degrees.  This bridge's interchange may be a little too tight for that (and it would be more expensive, of course), but at worst, I'd still take a conventional bridge/light setup over this.

Note that if you're willing to spend a little money, they have camera-driven lights that operate on a "left turn yield" basis these days; when traffic is light, the protected left never has to come on.

8

I think that if traffic was moderate to heavy, the newby would just follow the car in front of him.

And if traffic is very light, then it doesn't matter what the newby does.

9 RE:  Jeremy Bowers -

Several miles northeast of you on the map is a Giant circle.  It's not labeled on the google maps, and the street names seem very pedestrian.  And googling "giant circle Michigan" does turn up anything, and Sloan Lake doesn't give much either.  What IS that?!

10 Ach, I was intrigued by that, and tested my Google-Fu.  It's the General Motors Milford Proving Ground.

11 One drawback with the diverging diamond, other than what's already been mentioned, is that truckers can't roll through.  I saw a discussion of this type of intersection a couple weeks ago and there was a link to a discussion on a trucker's forum.  One guy implied that for his company, making a U-turn is pretty much a fireable offense (well, I think he said it would be treated as an at-fault accident.)  I believe the problem is that if you mistakenly get off at an exit where there's one of these, normally you could just go straight on to get back on the highway.  With one of these, you are forced to turn, and unless there's a rotary wide enough to feed an 18-wheeler, you're going to have to make a U-turn in a parking lot or something.

12 My father works here, which is the Chrysler proving grounds in Chelsea, Michigan. Once when flying west I was able to use that to orient myself and figure out which house was my in-laws from the air, though my parent's house was under the floor to me.

Re not liking roundabouts, your opinion is certainly not unusual. I've often wondered just what sort of fireworks were necessary to get that design past the relevant political bodies. People don't like one, and somebody actually proposed, championed, and won on a three-circle interlocking design. Somebody had guts.

From an abstract engineering point of view, I get it; it' s difficult to imagine what combination of lights would work there, since in addition to a major road + highway there's also the major original road the highway ended running next to, and it's a complicated little interchange. But still, I've often wondered what the story was.

13 They're in love with traffic circles here.  My town is working on their second.  They like them because they slow traffic down, eliminate traffic lights and left turns, and they can make a nice little park in the middle that nobody can use that they can spend money on "Art installations" for.  (The first one has a giant wooden fish or something.)

When I lived in South Jersey, well, the circles there are notorious.  Once the roads are major enough that you need traffic lights to meter traffic into the circle, then the circle no longer performs its function.  I think they finally got rid of one of the worst, but I haven't been there in over a decade.

14 Alexandria, La. used to have two; a very large one in the south end of the city, and a very small one in the north.  I don't mind the large one except at rush hour, but I am very glad they got rid of the smaller one; it was suicidal in my opinion.

15 The main problem I can see with it is the near-total inability to expand the highway's bandwidth in the future. A straight highway you can add another lane to fairly easily if the area turns out to be popular enough to need it. I have no idea how they could expand the crisscross, short of taking the entire thing down and rebuilding it from scratch.  

16 http://maps.google.com/maps?q=34.872091,-82.224598&ll=34.872095,-82.223541&spn=0.00349,0.004163&sll=37.0625,-95.677068&sspn=23.875,57.630033&num=1&t=k&vpsrc=0&z=18
This probably makes more sense for an intersection like that--no confusing crossover, and, IIRC, only _one_ set of lights.  It suffers the same drawbacks as the original one, though:  no rolling through the exit.
Here's an intersting variation: http://maps.google.com/maps?q=34.872091,-82.224598&ll=33.036624,-96.704825&spn=0.001783,0.003103&sll=37.0625,-95.677068&sspn=23.875,57.630033&num=1&t=k&vpsrc=6&z=19
This took about 3 years to build, heh, since they completely tore down the bridge over a major commuter highway, and redid the entire intersection.  One interesting thing is it has a Texas U-turn; the feeder ramp coming off the US 75 splits so you can make the U without going through the intersection.  The frontage road on the highway stays at ground level; the ramps to the overpass are elevated and between 75 and the frontage road.

17

Tatterdemalian, expanding a bridge is a pain even if it's straight through. You don't just "add a lane", you tear the bridge down and build a new one.

It wouldn't be any different if it was a criss-cross.

18 Not necessarily; I have seen it done where a lane was added.
  However, I think it was designed for the potential expansion in the first place.

19

No rule is absolute. But usually expansion of a bridge requires rebuilding.

When I was a little kid, we lived in Portland and my grandparents lived in Salem. When we wanted to go see them, we took US 99E.

Then they started building Interstate 5, starting from Portland going south. Originally it was 4 lane, and after a while it reached Woodburn. After that, we would use it to reach Woodburn, and take 99E the rest of the way.

Ultimately they reached Salem, and we didn't have to bother any more.

But traffic on I-5 built and built and the state decided to widen it. Only problem was, all the overpasses were too narrow. So they ended up tearing down and rebuilding nearly all of them. The new ones are a lot wider than they need to be for 6 lanes; they learned their lesson and made sure they weren't going to have to do it a third time.

They also had to rebuild the I-5 bridge over the Tualatin river (at Tualatin) and over the Willamette river (at Wilsonville). It wasn't cheap, and it took a long time.