# Why does traffic come in swarms on interstates? **URL:** \ **Category:** Factual Questions\ **Created:** [October 4, 2005, 12:46pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725 "2005-10-04T12:46:54Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Engywook](https://avatars.discourse-cdn.com/v4/letter/e/bc8723/32.png) [@Engywook](https://boards.straightdope.com/u/Engywook)\ **Post date:** [October 4, 2005, 12:46pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/1 "2005-10-04T12:46:54Z")
Subject pretty much says it all… on those long stretches of interstate between cities, where there’s only the occasional exit and the highway’s only two lanes, you’ll see a cluster of vehicles, and then nothing for a mile or so, then another cluster, then nothing… and so on. It becomes most noticeable when you’re looking across the median at the other barrel. So why do vehicles tend to gather in caravans? And to hijack my own question (no, I didn’t really want to start two threads about highways) - why do leadfoot drivers in LA pass on the right ? Even if I’m doing 75 mph in a 65 mph zone, with a respectable amount of space on either side of me, lots of these guys choose to whip by me at 90 on the right. In fact, it seems that the second lane from the right is reserved for those guys who careen around every other vehicle on the freeway. Seems the left is safer when it’s available… ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [October 4, 2005, 12:55pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/2 "2005-10-04T12:55:20Z")
> [@Engywook](#): > > Subject pretty much says it all… on those long stretches of interstate between cities, where there’s only the occasional exit and the highway’s only two lanes, you’ll see a cluster of vehicles, and then nothing for a mile or so, then another cluster, then nothing… and so on. It becomes most noticeable when you’re looking across the median at the other barrel. > > So why do vehicles tend to gather in caravans? I always thought it was because of the way individual drivers would tend, if not constrained by other nearby vehicles, to drive at slightly different speeds. Two or three vehicles travelling at a fairly moderate speed side by side (or nearly so) can serve as the nucleus of a cluster. Any vehicle coming up more quickly from behind will have to either navigate through the cluster, or will become part of it. If the cluster catches up on stragglers ahead of it, then each vehicle in the cluster will have to pass that straggler, or the straggler becomes part of the cluster. (Either slowing the cluster down or accelerating to match its speed.) Does that make any sense?? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 4, 2005, 1:11pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/3 "2005-10-04T13:11:26Z")
> [@Engywook](#): > > And to hijack my own question (no, I didn’t really want to start two threads about highways) - why do leadfoot drivers in LA pass on the right ? Even if I’m doing 75 mph in a 65 mph zone, with a respectable amount of space on either side of me, lots of these guys choose to whip by me at 90 on the right. In fact, it seems that the second lane from the right is reserved for those guys who careen around every other vehicle on the freeway. Seems the left is safer when it’s available… When I lived in L.A. I would pass on whatever side gave me a ‘tactical advantage’. Given a choice of passing on the left but seeing that I’d either have to slow down or pass again or passing on the right with ‘clear sailing’ ahead, I’d pass on the right. Since people _do_ travel in packets (in Driver’s Ed. the instructor called them ‘packet drivers’) often the best way to get round them was to pass on the right. I’m a ‘tactical driver’. That means I look well ahead to see what people are doing and plan my moves accordingly. I’ve often thought of it (since I spent so much time on L.A. freeways) as a game of chess. A less obvious move may be more advantageous than an obvious one. And many, many times the correct move was to not change lanes at all. When traffic is stop-and-go it makes little or no sense to change lanes, so I’d stay put. Of course on the motorcycle things were different. It’s useless to look too far ahead when you’re splitting lanes because the dangerous drivers are the ones who are right ahead of you. Or right beside you. Best practice there is to maintain a safe speed between the leftmost lane and the one to the right of it and stay there until you come upon a carpool lane so you can get out of it, and keep a weather eye out for drivers who are immediate factors. [sub]Incidentally I’m no longer a ‘tactical driver’ because of high gas prices. I’ve taken to driving in the slow lane at the posted Truck speed limit. Traffic up here is generally not so heavy as well.[/sub] ---
**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\ **Post date:** [October 4, 2005, 1:12pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/4 "2005-10-04T13:12:32Z")
Similar to cyclists or gazelles - it is easier to ride as part of a peleton. Not only are you saving energy on wind resistance, you are able to pace yourself as part of the cluster and less likely to be busted for speeding. BTW, I alway drive at a constant speed on single lane roads that have a traffic jam that is "catapillering’ along. I do this to pace out the traffic, thus preventing the stop-start traffic mentality of the traffic jam. Then recently I came across [Bill Beatys](http://www.amasci.com/amateur/traffic/traffic1.html) article on this phenomenon - great to see someone else doing the same thing. > [@Engywook](#): > > why do leadfoot drivers in LA pass on the right ? Dunno. In Australia, Plumbopods pass on the right, but the road rules are arse up over here. ---
**Author:** ![antechinus](https://avatars.discourse-cdn.com/v4/letter/a/fbc32d/32.png) [@antechinus](https://boards.straightdope.com/u/antechinus)\ **Post date:** [October 4, 2005, 1:15pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/5 "2005-10-04T13:15:42Z")
> [@Johnny L.A.](#): > > When I lived in L.A. I would pass on whatever side gave me a ‘tactical advantage’. Given a choice of passing on the left but seeing that I’d either have to slow down or pass again or passing on the right with ‘clear sailing’ ahead, I’d pass on the right. Since people _do_ travel in packets (in Driver’s Ed. the instructor called them ‘packet drivers’) often the best way to get round them was to pass on the right. Isn’t it illegal to pass on the right? ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [October 4, 2005, 1:19pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/6 "2005-10-04T13:19:55Z")
> [@antechinus](#): > > Isn’t it illegal to pass on the right? Generally, not on multi-lane highways in the US, or even to pass on the left in New South Wales (though drivers are more likely to drive in the left-most lane there than they are in the right-most lane in the US) ---
**Author:** ![The\_Chao\_Goes\_Mu](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_chao_goes_mu/32/3191_2.png) [@The\_Chao\_Goes\_Mu](https://boards.straightdope.com/u/The_Chao_Goes_Mu)\ **Post date:** [October 4, 2005, 1:20pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/7 "2005-10-04T13:20:52Z")
> [@antechinus](#): > > Isn’t it illegal to pass on the right? Depends on what state you live in. In Ohio, it’s considered bad driving etiquette but it’s not illegal. However, in New York, it’s illegal. I don’t know the status of the other states. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 4, 2005, 1:21pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/8 "2005-10-04T13:21:42Z")
> [@antechinus](#): > > Isn’t it illegal to pass on the right? Whether it is or isn’t, it’s impossible to enforce. But according to the [DMV](http://www.cal-driver-ed.com/dmv-handbook/20.htm): > [@](#): > > You may pass on the right only: > > • If an open highway is clearly marked for two or more lanes of vehicles moving in your direction of travel. > > • If the driver ahead of you is making a left turn and you do not have to drive off the roadway. Never pass on the left if the driver is signaling a left turn. (Be careful. Sometimes drivers use the wrong turn signal.) [Lane-splitting is legal in California](http://www.chp.ca.gov/html/answers.html): > [@](#): > > Can motorcycle riders “split” lanes and ride between other vehicles? > > Lane splitting by motorcycles is permissible but must be done in a safe and prudent manner ---
**Author:** ![China\_Guy](https://avatars.discourse-cdn.com/v4/letter/c/779978/32.png) [@China\_Guy](https://boards.straightdope.com/u/China_Guy)\ **Post date:** [October 4, 2005, 1:30pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/9 "2005-10-04T13:30:19Z")
If you are driving a car slower than the flow of traffic, the swarms that come up and overtake you are technically referred to as “wolf packs” 🆒 ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [October 4, 2005, 1:44pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/10 "2005-10-04T13:44:58Z")
> [@antechinus](#): > > Dunno. In Australia, Plumbopods pass on the right, but the road rules are arse up over here. I friend of mine returned from a visit to Australia. When asked if he would recommend it for others he said something along the lines of: Oh yes, it a great place, lots to see and do and if you listen real careful like, you can understand almost 80% of the language. ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [October 4, 2005, 1:51pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/11 "2005-10-04T13:51:49Z")
> [@The Chao Goes Mu](#): > > Depends on what state you live in. In Ohio, it’s considered bad driving etiquette but it’s not illegal. However, in New York, it’s illegal. I don’t know the status of the other states. It’s certainly legal to pass on the right in New York under certain situations. [http://www.nydmv.state.ny.us/dmanual/chapter06-manual.htm#pas-rig](http://www.nydmv.state.ny.us/dmanual/chapter06-manual.htm#pas-rig) > [@](#): > > You may pass on the right: > [ul] > [li] When a vehicle ahead is making a left turn.[/li][li] When you are driving on a one-way road that is marked for two or more lanes or is wide enough for two or more lanes, and passing is not restricted by signs.[/li][/ul] On the interstate, passing on the right is perfectly fine. ---
**Author:** ![Spiff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/spiff/32/9852_2.png) [@Spiff](https://boards.straightdope.com/u/Spiff)\ **Post date:** [October 4, 2005, 2:02pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/12 "2005-10-04T14:02:44Z")
**Engywook** , you’ve answered your own question. Even if you’re “doing 75 mph in a 65 mph zone,” you should only be in the left-most lane if you are actually passing other vehicles. And by passing, I don’t mean slowly overtaking them in the space of a mile or two. Pass 'em, and get out of the left lane. Even doing 75 mph in a 65 mph zone, there will still be vehicles wanting to go faster. It’s a sad fact, but it’s true. Left-lane drivers who are not actively (and quickly) passing other vehicles cause “wolf packs” (also the term my driver’s ed teacher used). Even if they are driving at or above the speed limit. ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [October 4, 2005, 4:03pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/13 "2005-10-04T16:03:43Z")
Perhaps you are encountering a [traffic wave](http://www.amasci.com/amateur/traffic/traffic1.html). ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [October 4, 2005, 5:53pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/14 "2005-10-04T17:53:15Z")
Besides the standing waves that **Duckster** spoke of, there are those cause by one slow driver. In a 65mph interstate, two lanes per side, you’ll have the right lane mostly filled with folks going about 71mph. The left lane is mostly filled with folks going 75 and faster. That all works pretty smoothly. But there’s an occasional car or truck going 65. Everyone in the right lane is going to have to get into the left lane to pass the slow leak. So, you have a big clot slowly sliding past the obstruction. It may take an individual car 5 miles to get through the clot. ---
**Author:** ![Engywook](https://avatars.discourse-cdn.com/v4/letter/e/bc8723/32.png) [@Engywook](https://boards.straightdope.com/u/Engywook)\ **Post date:** [October 4, 2005, 6:55pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/15 "2005-10-04T18:55:59Z")
> [@Spiff](#): > > **Engywook** , you’ve answered your own question. > > Even if you’re “doing 75 mph in a 65 mph zone,” you should only be in the **left-most lane** if you are actually passing other vehicles. And by passing, I don’t mean slowly overtaking them in the space of a mile or two. I can see how what I said left room for that interpretation. But I’m talking about driving in about the third lane from the right when there are five lanes available, as is true in LA (where it’s not generally true that inner lanes are faster than outer lanes). > [@chrisk](#): > > Two or three vehicles travelling at a fairly moderate speed side by side (or nearly so) can serve as the nucleus of a cluster. Any vehicle coming up more quickly from behind will have to either navigate through the cluster, or will become part of it. That makes a lot of sense. Sort of interesting… like planet formation. Testable, too - on I-15, near the Nevada border, you come over this mountain and can see 15 miles of highway stretching out toward the “town” of Primm. Should be ample space to observe changes in traffic patterns. And thanks for the link, **Duckster**. ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [October 4, 2005, 10:26pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/16 "2005-10-04T22:26:02Z")
This is actually a much harder question than you probably anticipated. Traffic flow science is an active field of study with much controversy. Many believe that it is related to fluid dynamics most closely. Google: “traffic flow theory” or “traffic bunching” to get a list of many articles and discussions. Here is a sample article: [http://www.amasci.com/amateur/traffic/traffic1.html](http://www.amasci.com/amateur/traffic/traffic1.html) ---
**Author:** ![Nic2004](https://avatars.discourse-cdn.com/v4/letter/n/4491bb/32.png) [@Nic2004](https://boards.straightdope.com/u/Nic2004)\ **Post date:** [October 4, 2005, 10:35pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/17 "2005-10-04T22:35:22Z")
In the field of Traffic Engineering the groups that are standing still, like at a traffic signal, are called queues (pronounced _Q’s_) The one’s moving down the highway are called platoons (pronounced _platoons_ 😉 ) ---
**Author:** ![Epimetheus](https://avatars.discourse-cdn.com/v4/letter/e/c4cdca/32.png) [@Epimetheus](https://boards.straightdope.com/u/Epimetheus)\ **Post date:** [October 4, 2005, 10:43pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/18 "2005-10-04T22:43:44Z")
Just want to chime in with my observations. I drive a 126 mile stretch home from college, there and back, several times a month. I tend to be a tactical driver as well, looking ahead, looking behind, and caculating and expecting other peoples possible moves. The highway (1-70) between Columbia, Mo and Kansas City is two lanes, and is heavily populated with large trucks. This may skew my observations compared to driving in commuter traffic. The clots mentioned above are quite frequent, and tend to happen most with semi trucks, which like to “pass” each other in rates that take 5-10 minutes to complete. Another problem is on-ramps. You will notice traffic tends to slow down at every on-ramp, and waves are formed as fresh clots are formed and re-form as they come to the next on-ramp. I am firmly convinced that highways would be much safer without semis. I can say without exxagerating, that I see somebody almost get run off the road by a semi, or nearly get hit myself by one, every time I drive home, and every time I drive back. They are by far the majority of traffic problems in the stretch that I take, and they seem to like to try and run people off the road. Of course, they are vital to our infrastructure, so all I can do is feeblely complain and flip em off when they try to run me down. ---
**Author:** ![rjk](https://avatars.discourse-cdn.com/v4/letter/r/ed655f/32.png) [@rjk](https://boards.straightdope.com/u/rjk)\ **Post date:** [October 4, 2005, 10:47pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/19 "2005-10-04T22:47:02Z")
Anybody notice that the same page was linked three times in this thread? It’s the one I was thinking of when I came in here. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [October 4, 2005, 11:40pm UTC](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725/20 "2005-10-04T23:40:02Z")
> [@antechinus](#): > > BTW, I alway drive at a constant speed on single lane roads that have a traffic jam that is "catapillering’ along. I do this to pace out the traffic, thus preventing the stop-start traffic mentality of the traffic jam. Then recently I came across [Bill Beatys](http://www.amasci.com/amateur/traffic/traffic1.html) article on this phenomenon - great to see someone else doing the same thing. That’s what I always _try_ to do, since I drive a stick-shift and it’s annoying to have to constantly shift; I’d prefer to do a contant 15mph than have to go in spurts and stops. Except that if I leave too much space in front of me, some imbecile determined to get one more car length ahead will pull in front of me, making me slam on my brakes. [Next page](https://boards.straightdope.com/t/why-does-traffic-come-in-swarms-on-interstates/324725.md?page=2)