Why big trucks block lanes: variances in speedometers and the law that says once you slow down you keep going slower and slower…

June 8, 2009

Do you find it annoying when two big trucks occupy the lanes in front of you on the interstate and you can’t get around them?

Well, so do I.

And I spent 12 years over the road behind the wheel of a big truck.

While I cannot say that I know the complete solution for this problem, I can say that what causes it is pretty obvious.

One truck is usually going about one mile per hour faster than the one ahead of it, so the driver goes to pass, but that takes quite awhile. And if the driver in the slower truck inadvertently or on purpose speeds up, it takes a lot longer and may never happen at all.

This subject came to my mind when I read what appears to be a new regular column in my local newspaper by a local Highway Patrol officer.

He began his column by laying out the scenario:

“You’re traveling down a two-lane freeway in the fast lane (known to law enforcement as the no. 1 lane) with your cruise control set at 70 mph. Approximately a quarter mile ahead of you are two commercial big rigs in the right lane (no. 2 lane). The big rig that had been following the other moves into the no. 1 lane. Now for the next few miles you sit there and simmer as the big rig slowly passes the other truck.”

So far, so good. He described the phenomenon we all have experienced. But that is just about where he left it. Oh, he went on to note that in California the speed limit for big rigs and other vehicles towing trailers is 55 mph (compared with 70 mph for other vehicles on most freeways). He also went on to say that in California the law is that regardless of what the speed limit is, slower vehicles, even if they are traveling the speed limit, have an obligation to go to the right and let faster traffic pass.

Well that might answer the question of where the law is for the so-called “speed monitors” who travel the posted speed limit but refuse to get over for cars going a little (or a lot) faster, but it does not address that first situation he described about the trucks.

Well here it is: on relatively flat ground, most truckers are going to be going as fast as they can or as fast as they can get away with (kind of like most other drivers). In California and other states with a split speed limit, 55 for trucks and 70 for cars (in California), there is a built-in problem in that no matter where a truck is, it is going to be holding someone up.

But the problem we were looking at originally and I will zero in on is those two trucks in front of you hogging the lanes in what looks kind of like a race of two turtles.

While in California the truck speed limit is 55, the average big rig is probably traveling about 60 mph – I don’t care what anyone says, that is the de facto speed limit for big rigs. Some truckers are in a hurry because they are paid by the mile and my experience tells me that 5 mph difference can make a big difference in pay over a pay period. They may be also pressured by dispatchers to get a load delivered or picked up by a certain time (you could ignore the pressure, but who wants to lose his or her job or get a short pay check?). And then 55 is really awful slow.

But while I have waited this long to get it out, the real problem here is some kind of physics or traffic science problem I can’t explain, but I can describe. I used to think how foolish it was of me to pass another truck when I was only going a tad faster than it was, and I also knew that I risked getting a citation. So, sometimes I would slow down a little and drop back with the intention of resuming at least a speed of 55 mph (not 60 – didn’t want to catch up with that slower truck again). But what will happen every time is this: after everything settles down I’m going 45 mph, maybe behind some partially disabled car. If I speed up, I’m back behind another big truck. That does not work and is not good for fuel mileage (constantly changing speeds) or even safety.

The only solution is to do one’s best to keep running at a steady, but safe (and legal) speed and pass when one must and one thinks the job can be done in a relatively short length of time.

I think split speed limits amount to accidents waiting to happen, but except out on the open desert, 70 is probably a bit fast for big rigs (and I know some will do it and faster even so), but 55 is a tad slow when the rest of the traffic is going at least 70.

As far as one truck passing the other, even if all the truckers tried to do the posted speed limit, such as 55, not all speedometers agree with each other (you’d be surprised at the variation), so there would still be the situation of one truck going a tad faster or slower than the other. Yes you can drop back, but then that causes a chain reaction in all the traffic and the truck driver winds up going not just 55, but 45, like I said (and I don’t quite know why).

The only answers are common courtesy and obeying the law on the part of all drivers no matter what type of vehicle they are driving.


Speed limits vary by state. Some trucks go extremely fast. But most big rigs owned by companies, as opposed to independent drivers, are electronically governed anywhere between 59 mph and 65 (especially on the West Coast). Why not 55? Don’t know if it is true, but the old story is that at least one company did cut its trucks down to 55, but the Highway Patrol complained they were going too slow, holding up traffic (I think that may have been in another state, not California, if it’s even true).