Just got back from a road trip. On occasion, I would see 2 trucks driving side by side at the same speed. The effect would be open road ahead of them and a huge backup behind them. Does anyone know what the rationale for this is? Or the effect on traffic? Does a “moving wall” driving at a constant pace lessen the “stop and go” effect?
This is a different scenario than when trucks team up to force folk to merge before a lane actually ends - which is an entirely different issue.
As to the effect on traffic (I’m currently taking a course in Transportation Engineering), these trucks will create a shock wave in the traffic stream (likely travelling opposite in the flow direction). Incoming vehicles will begin to “platoon” behind them. Once they get out of the way, another shock wave will release the plug, and it will take a matter of time to dissipate the platoon.
If there are two lanes on the highway, my understanding is the left lane is for passing and the right lane is for slow-moving vehicles. So the truck in the left-hand lane should have either passed the other truck or voluntarily return to the right-hand lane, at least until other vehicles had overcome them.
If there were signs posted that trucks or slow-moving vehicles should keep right, I suppose a trooper could pull over the trucker for a violation.
Here’s a nice page with animations on “traffic waves”: http://www.amasci.com/amateur/traffic/traffic1.html
It seemed to me that it would adversely effect the smooth flow of traffic.
Any truckers here who might be able to shed some light on why truckers would do this? Just because they can?
I’m not a trucker, but from listening to CB radio I gather that some truckers do this deliberately in the belief that it will help the flow of traffic. If I understnd correctly, they think that slowing down the traffic behind them helps break up traffic jams ahead of them.
Any truckers want to shed some light?
I’m not a trucker, but I know a few…
This is done on purpose and supposedly it actually keeps traffic moving faster when there is an upcoming slowdown such as a construction site or accident. Consider that when a lane is closed for construction, there are always some drivers that will not merge into the open lane until the last moment. This causes people to have to stop to let them in and creates a bottleneck even though there were (or should have been) warning signs miles ahead of the closed lane. Eventually, this behavior multiplies and traffic slows to a crawl. The truckers are trying to get everyone slowed down and in order before there is a chance for a bottleneck to form. The idea is that driving slowly for a short while is better than stopping for even longer.
Also, if the traffic slowdown ahead involves an accident, it is safer for everyone if traffic has slowed well before reaching the accident site. Truckers make a living driving on highways as quickly as possible. I assume that if the technique did not work, they would not do it. They certainly don’t have time to screw around pissing off drivers.
-Rob
Now, now RGillen. I was trying to stay away from the merge now-or-later chestnut. Some might say the problem is folk merging earlier than they need to, and then not wanting to let folk in at the point the merge becomes necessary.
In the cases I saw last week, traffic headed south was pretty heavy all the way thru Tenn and KY. I guess a bunch of spring breakers were getting an early start to Fla on Good Friday.
I was not able to tell whether or not this action by truckers helped or hurt things. Traffic would be pretty heavy - even stop and go for a while. Then there would be an open stretch. Then a slow moving truck wall behind which traffic was bumper to bumper for up to 5 miles (we measured as we flew the other way at 75)!
Actually at least one state that I know of actually encourages this (PA) they have signs saying lane closed 1/2 mi - stay in lane till merge point.
And in my humble O that’s the way it should be!
Windfish, what a GREAT link!! For years, I’ve been thinking of traffic in terms of acting like a slinky, but to actually SEE that principle in action is a wonder!!! Fabulous!!
Windfish, what a GREAT link!! For years, I’ve been thinking of traffic in terms of acting like a slinky, but to actually SEE that principle in action is a wonder!!! Fabulous!!
The real problem (IMHO) is when cars shoot up an empty lane (passing slow moving traffic) that is about to end then cut in at the last second making someone slam on their brakes. Are they better than everyone else, so they don’t have to wait? If everyone is merging at the same point all is 
As for trucks rolling slowly – it is easier to idle in a low gear and let a space in front of you grow and shrink than to keep pace with the stop and go traffic and be constantly pushing in a very heavy clutch. Of course as soon as the truck leaves more than 10 feet in front of him some car has to be in that spot, wrecking the whole plan. Perhaps when trucks ride side by side it is so they can keep a steady pace without cars contantly darting in front of them.
Thanks, Toaster52. The author of that site demonstrates what’s being described here as slowing down (actually, it’s increasing your headway that’s key) as you appoach a slowdown to help dissipate it for the people behind you. I encourage others to check out the animations on the site I linked to in my post way up above.
The truckers have the right idea for improving traffic flow, though I think that they may lack the authority to enforce their traffic calming manuevers.
The author of that site, William Beaty, is a regular poster here. I think he uses the handle BBeaty and he usually posts on electromagnetism.
The idea of traffic control is a good one, but I don’t think it answers the OP - in this example, the road in front of the trucks was empty, so they are not helping the flow of traffic.
This isn’t one of those extremely annoying incidents where one truck decides to overtake another on a two-lane road, even though their speed limiters are set with a difference of about 0.000000001 mph between them? This has to be one of the most irritating things on the road.
** Anachronism ** the hiway dept sets the merge point - if they wanted it a mile back then they would have closed it there. - who are you to change it.
As for the trucks - they are being selfish - they are reducing the jam for themselves but at the same time creating a worse backup behind them - just my humble O
So now you have a slinky moving back and a jam at the merge.
They should just shoot these people. - again just my humble O
Dinsdale: You’re right - I shouldn’t have brought up merging. I guess my point was that they aren’t out to specifically screw other motorists. The faster they get their delivery to where it’s going, the more money they get. It’s a weak argument, but I’m assuming that if it was bad for traffic flow, they wouldn’t do it. The road ahead was empty for now, but truckers are great at warning each other when something is coming. Besides, if they drive one behind the other, they each improve gas mileage by drafting. There must be some advantage (for them, at least). I agree with Anachronism regarding idling in low gear. It takes a lot to get a tractor-trailer started/stopped. The situation described by sirjamesp sounds likely, too.
Thanks for all the responses.
I have read a few interesting articles recently about applying fluid mechanics to traffic. Unfortunately, such considerations tend to quickly make this poor lawyer’s brain hurt.
Altho the road was clear immediately in front of these trucks, a few miles ahead of them it was quite congested off-and-on for a couple of hundred miles. I do not know if the areas of congestion were static around exits and such, or if they slowly progressed down the road. If they moved, it is possible that the trucks were moving at a steady pace behind the congestion. If stationary, the trucks simply were taking their time approaching it. No need to “hurry up and wait.”
I do know that the traffic behind the trucks appeared very unpleasant. It was not well-ordered traffic smoothly proceeding at a unified pace. Instead, quickly behind the trucks traffic appeared at least as unpleasant as the jams further down the road. So the only folk who clearly appeared to benefit were the lead truckers and maybe a couple of cars behind them.
RGillen, it strikes me as possible that the truckers might be doing this with the best intentions, but they might be mistaken regarding the beneficial effects. Not singling out truckers here. Many people share and strongly advocate beliefs that may be contrary to objectively verifiable fact - or at least reflective of a selective recollection of anecdotal evidence.
I agree, that truckers have better things to do than go out of there way to hassle auto drivers. And I admit that I do not know any truckers. But is it possible that driving a big rig might create situations where the driver either consciously or subconsciously “throws his weight around” or takes it upon himself to enforce what he believes to be the law or proper driving etiquette?
Moreover, trucks are so obvious, yet essentially fungible to most motorists. It only takes a couple of encounters with inconsiderate truckers - admittedly the exception rather than the rule - to create a longlasting antipathy towards all trucks.
Yes Definatly. There are many truckers who have the best intentions and feel they have to ‘help’ on the road, even if their attempt is misguided and upsets other drivers. Of course they have no authority to do this, but these are usually they same guys who stop to assist stranded moterists, report drunk drivers, and mayby someday pull someone from a burning wreck.
Ummmm Ok 
I don’t want to change anything, but I thought they reason for giving you one or two miles notice was so traffic had plenty of time to merge over smoothly. If you are shooting past a line of cars in a lane that is about to end then cutting in at the last second you are disrupting the flow of traffic and IMHO ‘cheating’.
I use it to keep the
morons-who-don’t-look-further-ahead-than-the-bumper-ahead-of-them
out of the traffic jam caused by the
morons-who-don’t-look-further-ahead-than-the-bumper-ahead-of-them
until the
morons-who-don’t-look-further-ahead-than-the-bumper-ahead-of-them
figure out, once again, that two objects cannot occupy the same space at the same time.
Put another way: how big of a hurry are you in to get into the traffic jam?
and, a final hint:
#1 cause of clear-road traffic jams: tailgating. LEAVE ROOM FOR PEOPLE TO MERGE!!!
One typical goal is to erase stop-and-go traffic by driving at the average speed. This does not create a back-up, instead it creates a smooth flow. (If you’re driving a high-mass vehicle, you really don’t want to waste gas and brake shoes on silliness like those spontaneous waves of stop-and-go traffic.)
Of course if the truckers misjudge the average speed, then the empty space ahead of them will grow and grow continuously, and there will be a genuine traffic-clot behind them. But the proper technique is to drive just fast enough that the wave of stopped cars ahead of you is just breaking up when you arrive.
animation: “TRAFFIC WAVES”
As others said, another common goal is to erase a “rubbernecker slowdown” at an accident site. These slowdowns create artifical bottlenecks, and once they form, it doesn’t take any rubberneckers to keep them going. If the truckers can hit the “slowdown” with a huge empty bubble, then the slowdown evaporates. (Actually it gets spread backwards for miles, but certainly the close-packed nonlinear part gets erased, and any overall gain in flow would have to come from removing the nonlinear behavior and restoring smooth flow.)
animation: “PINNED SLOWDOWN”
After I wrote the “Traffic Waves” website, I started getting mail from truckers saying “hey, you just rediscovered what we already knew.”
Hey, does this feature actually work? …
animation: “TRAFFIC WAVES”
animation: “PINNED SLOWDOWN”