Why do lorries try to overtake on hills?

September 25, 2007 11:00 AM

Why do slow-moving lorries (trucks) invariably choose to pull out to pass another truck at the bottom of a hill?

I've just had a very slow journey across the UK and was amazed at just how many times a truck which could go at most 2-3 miles per hour faster than the truck it was following would choose to pull out and try to overtake the vehicle in front at the start of an incline.

The result, of course, is a queue of irritated car drivers who'd otherwise have breezed past the trucks as they lumbered up the incline.

So why do they do it? Is it a deliberate ploy to slow cars down? Do lorry drivers swap tall tales about the longest queue they've managed to cause that day? Am I just unlucky to find the idiots travelling the same road as I am?

posted by Lionel d'Lion to travel & transportation (15 comments total)

IIRC, a lorry of a certain weight/height can do a maximum of 56mph. There's a part fitted to the engine called a "governor" which limits the speed.

Yes, some lorry drivers are numpties. Some will do it just for a laugh.

I'm curious, though. Why not just overtake them? It's not like they're racing along. :D
posted by Solomon at 11:09 AM on September 25, 2007

I'm curious, though. Why not just overtake them? It's not like they're racing along. :D

Because there's only two lanes.
posted by EndsOfInvention at 11:20 AM on September 25, 2007

2 options: One, it doesn't matter about the hill - Lorries are generally up against the 56mph limiter anyway (ie have plenty of power to go faster if they weren't limited), and being 2-3 mph faster makes a significant difference to your day when you are driving for 8 hours. Getting past the slightly slower (calibration of speed limiters varies) trucks is essential wherever you are; hill or no hill, they pass when they catch another truck up.

two: They are hoping that the truck they are passing doesn't have as much power as their truck does, and is hoping that they will get the overtake over quicker as their speed bleeds off as they hit the incline.

Chances are, the truckers couldn't give a crap whether you get inconvenienced, they just need to get where they are going as fast as they can...
posted by Brockles at 11:20 AM on September 25, 2007

Confirmation bias.
posted by number9dream at 11:23 AM on September 25, 2007 [3 favorites]

From the HGV section on How Motorways Work: The 56mph monotony can only be relieved by pulling out into other vehicles.

Truck drivers are like every other kind of driver on the road - some are pretty damn good and considerate, some are arseholes, and some don't give a damn.

But each one will tell you that they have a right to be there and a right to pass someone going slower than they are. Even if it's just a couple of miles an hour up a hill.

Re the two lanes argument, on really steep inclines the motorway usually has a crawler lane for slower vehicles, though not always.
posted by Nugget at 11:24 AM on September 25, 2007

I agree with number9dream. These are the times that are most frustrating, and so they're the times you remember. I'd wager that they pass on or near hills the same amount that they pass anywhere else.
posted by chrisamiller at 11:26 AM on September 25, 2007

A lorry speeds up as it goes down a hill and reaches it's maximum speed at the bottom. Therefore, it's best able to pass a slower one when it pulls out to pass at the moment it's going fastest, just before it starts uphill again.
posted by KRS at 11:26 AM on September 25, 2007

I suspect they are going the speed they want to go for their weight and momentum. In hilly country, they barrel down one hill to gain momentum for climbing the next. If the guy in front climbs a hill 2 mph slower, the guy behind doesn't want to slow down and waste momentum, so he pulls out and passes him at 2 mph.

That's annoying to others, but, because they spend about a thousand times more time on the roads and get paid for it, I guess some truck drivers consider the roads to be theirs and car drivers to be amateur nuisances.
posted by pracowity at 11:28 AM on September 25, 2007

Any chance the trucks are drafting each other and switching at that moment, perhaps with the reasoning KRS gives?
posted by artifarce at 11:28 AM on September 25, 2007

Because there's only two lanes.

There might well be on a dual carriageway, but most motorways have 3 lanes. Besides, the way the question was asked implied that this only happens on hills.
posted by Solomon at 11:29 AM on September 25, 2007

A friend of mine was a trainer at a trucking company. Downhill, trucks will accelerate as much as possible, so they may in fact pass a slower truck. They do this to build up momentum for the next hill. To a trucker time is everything, they're allowed to drive so many hours per day and there are steep penalties for not being on time as there are for putting in extra hours behind the wheel to make up time.
posted by substrate at 11:30 AM on September 25, 2007

Keeping their momentum is the main answer. Not only with mass of the truck itself, but also with the oversized flywheel to keep the engine rev's higher. Truck engines are much less responsive than typical small vehicle engines.
posted by JacksonEsquire at 11:49 AM on September 25, 2007

I think I'm a victim of confirmation bias. Either that or I was projecting my "Don't pull out! Don't pull out! D'oh!" :-)

The journey was mostly on dual carriages, as has been deduced above. Maybe there is a good reason to take the M25 instead of the A14 after all.
posted by Lionel d'Lion at 11:50 AM on September 25, 2007

I think the key is that they're not accelerating. It's not that they want to pass the next one, it's that they don't want to decelerate.
posted by blue_beetle at 12:07 PM on September 25, 2007

Maybe there is a good reason to take the M25 instead of the A14 after all.

No such thing - I do a 120+ mile roundtrip on the A14 most days and at least you keep moving - as opposed to the M25 where I only ever seem to be going nowhere fast...
posted by koahiatamadl at 1:11 PM on September 25, 2007