When the traffic is light on the highway, I’ve noticed that cars tend to move in small groups of 4-6 cars, taking up all lanes and moving at almost the same speed. Then there’s a stretch of a couple of miles of empty road and then another group of closely packed cars, etc.
What causes these clusters of cars? Do people feel safer when there are other cars around?
I should mention that in Greece speed limits are not really respected and 99% of cars are not equipped with cruise control.
They are called “platoons” and are often a result of a group of vehicles having been stopped at a traffic signal and then proceeding on the green. Also, they may be forced to queue behind the lead vehicle when they do not have the opportunity to pass.
The stop lights are bunchers, and the groups are bunches. If, as they should, the faster cars move on.
The slower ones are stopped at the next light, the faster ones move on to the the second or third light.
This separates the sheep from the goats as long as the pokeys don’t block the way.
There is a whole academic field devoted to the study of traffic theory. The have found that it often acts in ways described by fluid dynamics more than other approaches.
Here is an interesting paper that addresses much of your question:
Clumping occurs for a number of reasons, but chiefly:
people are generally reluctant to overtake when they are moving only a very tiny fraction faster than another car, so a bunch of cars all travelling at very slightly different speeds tends to get hung up just behind the a marginally slower one.- there’s a sort of asymmetry in terms of what you can do in relation to another car’s speed; you can go slower than the car in front (and drop back), but it’s not quite so easy to go faster and overtake.
Overtaking prevents other cars overtaking, so while car A is passing car B, cars C, D, E and F are also waiting their turn.
I’ve heard about the fluid dynamics, like how the right lanes should move slower than the left lanes, as water moves in a pipe more slowly at the edges.
When I was in driver’s ed they called these “wolf packs” and told us to avoid them, because you’re more at risk to get into an accident. I can think of a few reasons people are in them, many have been listed, but others like not wanting to overtake the first guy incase a cop is gunning people up ahead (or the guy in front of you has a radar detector), or drafting to reduce gas costs, or not wanting to be the lead car because you hate having people tailing you.
I think there is also a tendency to match speeds with other cars, if they are close to the speed you are at. If I’m not using cruise control (and I seldom do) I will notice that I’ve placed myself a distance behind another car, and I seem to be subconsciously using that car as a way to set my speed.
Say you are driving at the highway speed limit of 65mph. Soon, someone will come up on you that was probably going 10-15mph over the speed limit. Not slowly creep up on you - come up fairly quick. They will camp out about 2-3 car lengths behind you, slow down to your speed and just sit there. Plenty of room to pass, but they are content to follow you. You can speed up and/or slow down and they will match your speed and maintain their distance. It’s amazing that someone who earlier was going 75mph to catch you is now content to sit right behind you and go 55mph with no desire to pass.
You change lanes and maintain your speed and the other car will once again slowly speed up and pass you, eventually going about 10mph faster than you. As soon as they come up on another car in front of them, they will slow down and follow them.
You will see them drive for miles, speeding up to catch a car in front of them, follow until the car changes lanes or exits, then speed up to the next car. I see it so often that I recognise a “follower” very easily.
A lot of people will come up on you, either already in the lane to your left, or will pull into that lane to pass you. They will get right along side you, ahead by about one to one-and-a-half car lengths, and then slow down and match your speed. They seem determined to get ahead of you, but only by a few feet and they are satisfied. They will even block the lane for faster traffic behind them just to drive along side you. They will vary their speed just enough to stay close to you so that other cars cannot pull over and pass between you.
Some drivers are just sheep looking for a herd to join.
Pack drivers, gaggles, followers… I prefer to call them liabilities, and try to avoid them as much as possible. It’s usually to first two yahoo’s in front who do the speed limit. But when they do the limit, it causes the two cars behind them to go 2mph less, and so the two behind them, another 2mph less. This causes frustration to the drivers at the back because they’re doing less then the speed limit, get frustrated, and try to over take all the “slow” drivers.
Incredible that this thread is on the Boards today. Yesterday afternoon I drove about 70 miles into NYC with both kids in the car. We were discussing the same thing. I try to avoid these groupings, and I told the kids that I blame these random tight bunches of vehicles on NASCAR. ( This was good for a laugh, and I was only half-kidding ).
It is completely frightening to watch 4-6 cars moving at 70 mph when there is no more than the width of a few bumpers between each car. Before NASCAR became so incredibly popular, most people never would have thought it cool or safe or acceptable lto drive in packs that closely on an open roadway. See, I’m not talking about rush hour- it was reverse rush. The road was open and flowing nicely in our direction. Despite this, drivers were pacing eachother with much less than one car length of distance in between. Back when I took driver’s ed, the rule was one car length for ever 10 mph.
The fluid dynamics thing is spot-on, I’ve seen documentaries on traffic flow analysis. I’ve always wondered if it would become a combination of fluid and swarm mentality behavior, if drivers while operating had dashboard monitor access to a bird’s eye-view of the roadway section that they are approaching.
I’m a little surprised at people talking about traffic signals and stop lights, since I think of the phenomenon as being on expressways where there aren’t such things.
I also think ParentalAdvisory’s argument doesn’t make much sense - but I think that as long as there’s some variation in speed among drivers I can see how these clumps would develop… people who tend to drive a little faster would come up on people who are driving at a medium speed, who are catching up to people who are driving a little slowly. When this happens all at one time, (which would be infrequent but not that rare,) you have the makings for a clump to form, and it would tend to grow from there instead of dispersing.
Interesting article, Cartooniverse. I agree that many people follow way too close and it causes a dangerous situation for everyone around them.
The problem here where I live is that, if you are on the highway running 55-65mph, and you try to keep 5 or 6 car lengths between you and the car in front of you ( or even just 3 or 4), that someone will have to pull in between you. The road can be clear of traffic, but they want that one spot between the cars.
If you back off to give yourself more room between you and the new car, another car will pull over into the space. It gets to the point, that if you back off for all the cars that pull into your “safety space”, and yet other cars keep pulling into the space, pretty soon you will find yourself back home sitting in your garage.
This does explain why I never have to fill my gas tank any more.
I agree with you. People Do slide in front of me. Since I sold the Nissan Quest and got a huge diesel Suburban, I drive with incredible caution and lower speeds. Even the wife has remarked upon this. The vehicle is so heavy that I fear skidding and losing control if I have to hit the brakes. Hence the increased distance.
There are times when one moves around traffic, and times when one stays in the right lane and lets traffic fly by. I typically get into NYC using a roadway that alternates between 50 and 55 mph. I get into the right lane and hit cruise control and unless I slide to the left just to allow a bunch of oncoming traffic to merge in, there I sit for about 40 miles.
Clusters form because some people are just extremely uncomfortable with having the freedom of an unconstrained empty roadway, I suppose. My mother is a pack driver and I’ve asked her this before and she claimed that “it’s more orderly” that way.
Personally I can’t stand having cars behind or in front of me and will actively merge into an empty lane regardless of speed.
A somewhat related question is - apparently common wisdom and some laws state that when you are traveling in a middle lane of a 3 lane roadway and need to overtake the car ahead of you, you merge left. Now, this makes sense if there are slow cars entering the roadway on the right but otherwise it seems to me, at least, a lot more dangerous to overtake on the left than on the right. In fact, if I had to order from safest to most dangerous it would be: Overtake on the right, overtake on the left, “undertake”* on your right, “undertake” on your left.
*By ‘undertake’ I am referring to merging to an adjacent lane, decelerating until the car that was behind you is ahead, and merging in behind them.
Why? I’m not saying you’re wrong, just wondering about your reasoning.
The primary reason it seems safer to me to overtake on the left rather than the right is because of blind spots. Drivers (especially truck drivers) have larger blind spots to their right than to their left. If you pass a tractor trailer on the right, you’re passing through a huge blind spot for the truck driver.
I tend to do that, but for a reason. I’ve gotten alot of speeding tickets in my life and this helps me keep myself from speeding. As long as I stay behind the person going the speed limit or a few over, it keeps me safe. As hard as it is for me to not pass them, it’s a hell of a lot harder to maintain legal speeds without someone impeeding me. But I’m sure that doesn’t describe the masses.
