Mailbag: The Crosstown in color and ghost jams
Posted on March 12th, 2008 – 6:54 PMBy Roadguy
Alert reader Joel had a bit of spare time on his hands recently, so he pasted together some diagrams of the I-35W/Crosstown project and added a little color to help clarify where and what all the new roadways will be. The helpful result can be viewed here. Joel writes:
It’s a 1MB+ file, so it’ll take a while to download. It’s also a PNG, which older versions of IE might have trouble with. 35W is red, 62 is blue, and streets with access to the highways are green. It’s pretty self-explanatory after that.
(When I first pasted in Joel’s quote, I somehow deleted an “s,” which made things “elf-explanatory,” which is something entirely different.)
As we know, even newly designed roads can’t stop all jams, and alert reader Rico Suave points us to this study out of Japan, complete with video. It looks at the “shockwave effect,” which is when a slowdown of no apparent cause moves through freeway traffic — according to the article, it turns out they’re “typically triggered by a single driver slowing down” and “human error.” Roadguy has always wondered about “ghost jams,” and now he knows: It’s the power of the individual to affect society. Lucky us.
23 Responses to "Mailbag: The Crosstown in color and ghost jams"
I have suspected fro some time that what I face heading east o n I-94 any time after 3:30 p.m. is a shockwave jam. I believe that it is caused by folks not wanting to or not knowing how to deal with traffic coming in from 61 slowing down in reflex and/or moving to the middle lane, causing a slow down there that then ripples over into the left lane. Traffic generally starts to ease by the time one gets to the White Bear Ave. exit in spite of the fact that there aren’t that many people exiting there.
Judy, I’m also convinced that’s what happens on the Crosstown heading west through Edina in the mornings. The curves east and west of France Ave and the two successive entrances at France and Valley View cause just enough drivers to get nervous and pause and the entire highway can be backed up to Xerxes, Penn, or even worse some mornings.
Ironically enough, once the Commons is finished (self-promotion: neat color map, huh?) that particular “ghost jam” on 62 is only going to get worse as a higher volume of cars gets through the Commons at a higher rate of speed. I can’t wait…
Nice work on the map Joel. For folks interested in seeing just how much space this freeway expansion is taking up, MnDOT produced a video simulation that shows the pavement virtually eating up land. A soundtrack has been added:
http://www.youtube.com/watch?v=61cGOXbqrgQ
The “shockwave effect” doesn’t happen with steel wheels on fixed steel rails. Rail transit also allows commuters to work, socialize or even catch a few extra Z’s during their trip. Needless to say, rail can carry many more people while using a lot less land than freeways.
A few years ago I played with a computer simulation of shockwave jams. I believe it was an MIT student in fluid dynamics who studied the problem the same way people study rivers. In any case, the root cause was basically tail gaiting. When you were on the freeway, and driving with less than enough space for someone to change lanes in front of you without changing speeds, the conditions are right for a shockwave. Someone might change lanes to avoid an object or allow a car to merge from a ramp, and if that causes the person behind to tap the breaks, a wave has started.
If people drove with additional space, enough for a car to merge over, didn’t worry if someone changed lanes in front of them, and kept that space *all the time* (even if traffic stops) a lot of these jams would go away. Think about how much space you leave when all the traffic is doing 70. Leave the same space when you’re doing 20. And remember kids, it’s not a race and there are no points for passing cars.
Matty, awesome link. It’s amazing to see just how much space big highways take up. Every time I drive through that construction zone, I wonder when the first snow/ice-induced accident involving Peter’s Billiards will occur. Man, that’s going to be a tight fit.
Thanks to Joel for the map… that does help out. I’m just a little surprised by the entrance ramps that aren’t in the final design (from Lyndale to NB 35W, from Portland to WB 62 or SB 35W, and from 66th to EB 62 via NB 35W) — I hadn’t looked closely enough to see that they weren’t there.
Is someone making sure that MnDOT puts up signage on SB 35W to make it clear that you have to get on the EB/WB Crosstown exit to get to the exits for 60th Street, Lyndale, and Portland? I can see drivers easily getting turned around and lost because they won’t realize that.
I could have done without the screeching at the beginning, thank you……
I don’t know if its the same study Tim H refers to, but a couple years ago there was an interesting article about a group that was trying to model traffic jams. I want to say they were out of the Los Alamos lab in New Mexico. Anyway, the story described how they’d test their theories by sending drivers into rush hour to see if they could create traffic jams in free-flowing conditions by doing something stupid.
Someplace in my piles of stuff from MnDOT is the observation that 50-60% of all congestion is caused by “incidents”, not lack of capacity.
Our freeways today are moving more cars than the traffic engineers ever though possible. But it’s because of tailgating - drivers are simply becoming accustomed to driving ever closer to the car in front. At best, it works. At worst, you you get 50-75 car pileups as there’s no recovery space. In between, you get more ghost jams.
One idea that is working in Europe is variable speed limit signs so that a traffic management center can slow down the traffic approaching a ghost jam thus allowing it to clear more quickly as fewer cars pile into it. As sensible as that is, I suspect that that would be considered a violation of driver’s rights in this country.
Sorry about the lack of warning on the screeching, Dave G.
I came here to mention that most shockwaves are caused by tailgating, but others beat me to it.
To add, though, another way is to look at it like this: Car A can stay the same speed, but Car B catches up and starts tailgating. Car B slows down to make room again, while Car C has to slow down to accommodate for B and itself, thus slowing down more. Car D has to slow down to accommdate B, C, and itself, thus slowing down even more.
I remember on the Discovery Channel an experiment where Car A did exactly 45mph as controlled by computer. Car B would tailgate and back off. By the time the showed the 20th car, Car 21 was stopped! Remember, Car A was a constant speed through all of this. Eventually the end of the line would speed up and catch up again, to have it happen all over.
Sean: easy fix. Car A looks in their mirror, sees car B approaching and moves over a lane. Problem solved.
Matty and Other Mass Transist Freaks;
IMO it would help your agenda if you didn’t think of a road as ‘raping the land.’
Nobody thinks like that, except you ‘freaks’. It’s a road, get over it. You may not disagree with it, but since 95% of the folks want to travel over that road, you have been ‘voted out’.
“Sean: easy fix. Car A looks in their mirror, sees car B approaching and moves over a lane. ”
Causing Car E in that lane to have to slow to make room, lather, rinse, repeat.
if car E was maintaining proper following distance, there would/should be ample room to move over.
I do like the idea that many of you had about not tailgating, but the only way that is going to work is if we get cars that automatically drive themselves. It only takes one person to do something dumb on the highway, then we are all stuck. Here’s an incomplete list of everything I see as causing ghost jams:
Tailgating
Driving below the posted speed
Driving the posted speed in the left lane
Speed Traps (yes, cops cause ghost jams, I’ve seen it happen numerous times)
Drivers who are lost/don’t know what turn to take
Drivers who wait till the last minute to merge (huge one for me since I take 494 N to 55 West to get home to Rockford. Two spots that happens, and it amazes me when a car goes speeding around me in the right lane, only to merge 2 cars up, when they could have just merged in behind me. I like to wave at these people when I get up next to them at a stoplight)
How does car E’s proper following distance with the car in front of him make any difference at all when car A has to cut in front of him when A moves over to let Speedy Gonzalez Car B zoom by?
Easy. If car E is following at the safe/proper distance, car A should be able to slide over without affecting car E whatsoever. There should be no “cutting in”, just merging over. It’s done thousands of times a day.
If you’re following at a safe distance you don’t need to break when someone merges in front of you. All you do is let off the gas a bit to get a safe distance again, which dosn’t cause the waves that start a ghost jam. Google “Smith system” My wife is a school bus driver and swears by their way of making driving safe and sane.
E is following at a safe distance in Lane 2, A is in Lane 1. A moves over in front of E to let Morg and his band of left lane patriots through. E slows down for this because now his following distance has been drastically reduced. Whatcha missing?
To simply slow down, as Josh R pointed out, does not cause the shockwave that applying the brakes does. When I see brake lights ahead, I instinctively hit mine too if I’m close, whereas if someone merely slows down ahead by easing off the gas for a second, there is no net effect at all.
You know, it really doesn’t matter. This volleying back and forth is pointless, because you’re probably one of the people who drive in the left lane for no reason and refuse to move over and think anyone going faster than you is an idiot. I’d have better luck trying to talk to a rock.
Next subject!
Cars slowing down can’t cause a shockwave? Whoa. I have nothing more to say, your honor.
