General Question

Hawaii_Jake's avatar

Why am I in very heavy traffic, and then it suddenly disappears?

Asked by Hawaii_Jake (32726points) November 1st, 2016

It’s easy to understand how heavy traffic starts. There’s a section where the H201 and the H1 merge. That’s simple to comprehend, but what about when the traffic suddenly thins?

There is one exit where quite a few cars leave the freeway, but not that many. And poof! the traffic accelerates from 20 to 50 immediately.

OK, civil engineers, what’s going on? I have wondered about this my whole life.

While you’re at it, can you tell me why traffic is the worst on Thursdays?

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12 Answers

marinelife's avatar

I put it in the category of mysteries of the universe.

CWOTUS's avatar

I can’t tell you anything about Thursdays in Hawaii. Invite me sometime, and I’ll be happy to spend a few weeks working out some theories.

But I can offer some guidance on why traffic can improve even when the volume hasn’t changed so much. There’s a similar situation that I have to drive through – if I can’t avoid it – once or twice a month.

What happens in Connecticut frequently (at several highway exits that I’m very familiar with) is that the traffic leaving the primary road, which we agree is clogged for some apparently mysterious reason, also backs up at the exit itself – which may be normal because of the design of the exit or traffic on the road being exited onto. This causes slowdowns in one entire travel lane as it backs up from the mouth of the exit back into the primary road. Naturally, this causes some of the congestion as one lane comes to a near standstill, but wait! There’s more!

Since that travel lane is blocked and (one presumes) some of the people stuck in that lane aren’t planning to exit, they’re trying to turn back into the lanes that are moving. That causes sudden stops and slowdowns in the lanes that could, but for those drivers entering at 0 mph, keep moving at a decent speed. There’s even more than that, though.

Some of the knowledgeable drivers, knowing that the exit lane might be backed up for up to a quarter mile or more, stay in the relatively higher speed lanes (and slowing down) as they look for the infrequent openings in the exit lane to dive into and steal a march on some of the more patient drivers. That slowing also causes disruption in what could otherwise be a more even flow of traffic.

Once past the exit lane, all three of those conditions are past: no more slowed and stopped traffic at the exit, no more drivers stuck in the slow lane trying to turn into the travel lane, and no more drivers trying to ‘cheat’ their way in to the exit lane.

jca's avatar

I find if one idiot is going slow in the left lane, tapping his brakes or just slow in general, it will have a ripple effect on the whole span.

Hawaii_Jake's avatar

@CWOTUS I first began to notice how bad traffic was on Thursdays when I lived in San Francisco 25 years ago, and it’s not limited to the US. It was bad in Singapore and Sydney, too.

I like your illustration of how exiting traffic affects all lanes. A lot of cars exit at Punahou St., but not that many to enable me to increase my speed from 20 to 50 immediately. I still find it bizarre.

CWOTUS's avatar

Well, Thursdays are also Ladies’ Night at a lot of bars (and it’s the night of my once-a-month book club Meetup), so that could play a part. (My book club, a tiny part, admittedly.)

Blame it on the ladies.

elbanditoroso's avatar

Read this book. It will answer a lot of questions…
book info

Tropical_Willie's avatar

Okay lets try a simple answer, imagine a funnel in a bottle. You can get 15 beans a second through the funnel no problem. Gridlock strikes at 20 beans, only 18 or 19 beans a second can pass trough the end of the funnel after you reach the limit of 19 per second things start to back up.

In transportation the phrase I’ve heard is, “Too many cars and not enough asphalt”.

CWOTUS's avatar

That’s certainly accurate, @Tropical_Willie, and I suppose that most people are sadly familiar with that scenario, but @Hawaii_Jake‘s question was a good one about those times when the traffic for some unexplained reason seems to “magically” dissipate, even when the overall volume change is not that great. The scenario I described above is a real one that happens daily on some of the roads in this area.

Tropical_Willie's avatar

The reason is you go from 20 or more beans (CARS) to 17 (CARS), only a few cars get off at the exit, three to be exact. Now there is enough asphalt for the rest of the cars.

Call_Me_Jay's avatar

You can have smooth-running traffic or a traffic jam on the same road with the same number of cars.

The difference is following distance. People tailgate, and it causes a stop-start traffic jam. They rush up to fill any gap, only to brake and wait for the car in front to proceed.

If everyone maintains a good distance, everyone can move along a steady pace. This is one of the big benefits we will get from self-driving cars. They don’t have an emotional need to compete on the road.

From a recent Wall Street Journal article on the phenomenon:
There is a growing body of research finding that an individual driver, by preventing bottlenecks and maintaining a steady speed, can sometimes single-handedly ease or break up a traffic jam.

Mind the Gap
An individual driver with a calm, steady style can smooth waves of stop-and-go-driving before they expand into a traffic jam.

Disarm Aggressors
Keep a gap open in front of you so that when a road rager races to fill it, you won’t even need to tap your brakes. This prevents aggressive lane-changers from triggering jams.

Ease Exits
Encourage other drivers to merge into exit lanes in front of you, so they won’t have to slow down and block adjacent lanes waiting to merge.

Prevent Bottlenecks
When entering a congested zone where lanes are merging, stop pushing ahead Instead, open a wide gap and allow other drivers to merge into your lane without stopping.

The WSJ story is behind a paywall, but you can Select All and copy the text out of the page anyway.

Here is the guy who wrote the tips above. He’s obsessesed with the subject, and has some cool animated illustrations of the concepts. – William J. Beaty – Traffic Waves

Zaku's avatar

What @Tropical_Willie and @Call_Me_Jay wrote.

Some traffic jams, especially on highways, are not actually being held up by anything ahead of them at the moment, but are still going to take a while to unwind due to the effects such as @Call_Me_Jay refers to, and the number of cars involved. But once they do unwind, since there is a wide road ahead with no obstacles and a high limit, it can open up very quickly.

Call_Me_Jay's avatar

There is a section of expressway in town here that amuses and annoys me. There is a curve of about 40 degrees, like a hockey stick. And it’s a choke point.

The number of lanes does not change, there is no big entrance or exit – just going around the gentle curve jams things up. I have stopped trying to understand it and simply enjoy the release when I round the corner and traffic zooms ahead.

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