The science behind traffic jams...

Jstas

...it's all YOUR fault!



And all you people from L.A. that think the traffic is so bad? You didn't even make the top 10 so stop complaining.

coolsax

I'm surprised the Austin I35 corridor isn't in that top 10.. its pretty bad most days.

bobman1235

I think the problem with LA is that so much of it is highway so when it's congested, EVERYTHING is congested for a giant area, even if the congestion itself isn't the worst.

I've only been there a few times though, I'm not speaking from experience.

mhardy6647

I remember reading an article on this topic a long time ago in the San Jose Mercury News when we were house-hunting in the Silicon Valley (1989)... one of the interesting factoids: in heavy traffic, the net speed of vehicles in a growing traffic jam is actually negative; basically the length of the backup is increasing faster than cars are moving at the front of it. Consequently, traffic at speed isn't hurtling towards a dead stop; it's effectively hurtling towards cars that are backing up towards the oncoming traffic!

The article also noted that the throughput of vehicles (number of vehicles passing a given point per unit time) is maximal on a freeway at a speed of about 35 mph. At the time (1989) that was the average freeway speed in rush hour in the SF Bay area; i.e., the freeways were effectively saturated with traffic.

BlueFox

I always jokingly say one cause is to low speed limits. If the speed limits were higher then a given stretch of highway can accommodate more traffic in a given time period. Of course, the resulting wrecks from people who cannot drive at any speed will negate the savings.

bobman1235

BlueFox wrote: »

I always jokingly say one cause is to low speed limits. If the speed limits were higher then a given stretch of highway can accommodate more traffic in a given time period. Of course, the resulting wrecks from people who cannot drive at any speed will negate the savings.

I know you said jokingly, but that would only apply when traffic was moving at or near the speed limit, at which point there's already no traffic.... Once you introduced enough cars for the road to be congested, people wouldn't be going fast anyway, so it really wouldn't affect anything but non-peak transit times.

mhardy6647

Because humans' reaction times are fixed, and finite (at about a tenth of a second), faster speeds don't actually increase the throughput of traffic - the spacing of cars has to increase as traffic' speed increases, so the density of traffic has to decrease or the number of accidents will soar. That's why the optimum speed for maximum throughput (or density) is about 35 mph.

Interestingly, in terms of feedback-loop controlled, sensor-mediated "smart" automation, 0.1 seconds is a long time. I would imagine that computer-controlled freeways could permit much closer spacing between cars... imagine what it would be like to be "driving" down the highway at, say, 85 mph with two-foot spacing between cars!

megasat16

mhardy6647 wrote: »

Because humans' reaction times are fixed, and finite (at about a tenth of a second),

I contest the above statement. Human's brain and body doesn't respond equally from person to person.

I know first hand coz I response slower than most human. :biggrin:

mhardy6647

I hear you, but it is pretty much fixed for a given human :-) The 0.1 sec is an average; it's about right for the average person. That's where that old "catch a dollar bill" trick comes from.

BlueFox

mhardy6647 wrote: »

Because humans' reaction times are fixed, and finite (at about a tenth of a second), faster speeds don't actually increase the throughput of traffic - the spacing of cars has to increase as traffic' speed increases,

Don't tell that to NASCAR. Or rush hour freeway drivers in CA. :biggrin:

Jstas

mhardy6647 wrote: »

...imagine what it would be like to be "driving" down the highway at, say, 85 mph with two-foot spacing between cars!

Sounds like The NJ Parkway or AC Expressway. Or, even more fun, The Surekill Deathway (Schuylkill Expressway/I-76 through Philly). That's elevated with concrete walls on both sides. On the Surekill, speed limit is 50 in most places. If you're doing 70, you're slow. Someone will pass you like you're standing still. I've been on that road on many occasions during rush hour traffic and there is a mass of cars with minimal spacing between all moving at 65 MPH. Unless there is an accident somewhere, traffic doesn't usually move much slower than that. I wish I was exaggerating too.

The NJ Parkway up near Newark/Jersey City is 6 lanes wide in both directions in some places and moves the same way. Those lanes are tight too. A typical full-sized pickup truck can have less than a foot of space between a door handle and the lane lines in some spots.

I'd offer the Cross-Bronx as an example as well but you said traffic would actually be moving and that doesn't happen on the Cross-Bronx.

mhardy6647

FWIW, my wife has always told me that the Long Island Expressway is the worst she's seen in terms of high-speed, bumper to bumper traffic.

fatchowmein

Today's jam was caused by a Range Rover with a flat front driver side tire doing 50 in the "fast lane" during rush hour traffic on a major highway.

mdaudioguy

Thanks for posting this. It validates the thoughts I've had based on my observations in traffic in and around the I-95 corridor for more than 20 years. The real question is - What is government going to do about it? Yes, government. While individual driver behavior is unpredictable, driver behavior as a whole seems to be largely predictable. Government agencies need to stay on top of changing traffic flows and patterns and make adjustments as necessary. Individuals cannot do this. Existing traffic flows often exceed what I see as the design limits of some stretches of roads.

There's so much more to this, but I've apparently been living under the mistaken impression that "somebody" should be regularly assessing the situation and making adjustments. The majority of backups are consistent and predictable. Yeah, that would be expensive, sure, but how much gas are we wasting every day in stop and go traffic? And, how much money, sleep, and time with my family, does this traffic cost me? I really don't think the government will help, but it is a nice dream. Anyhow, off to battle the monster once again! Thank goodness for my car stereo! The upgrades were worth it!

jimsvm

:confused Just drove from Ont to SC. People totally disregard the term "Slower traffic use right lane". Traffic would flow alot better.

Jstas

jimsvm wrote: »

:confused Just drove from Ont to SC. People totally disregard the term "Slower traffic use right lane". Traffic would flow alot better.

No, it wouldn't. Because then you want the majority of drivers already clogging up a 3 lane highway to cram in to two lanes where there is even less room just so some Canucklehead can break the sound barrier on their way to the beach.