One of the most common things I am asked when I tell people I am a lorry driver is 'why do lorry drivers insist on overtaking each other on dual-carriageways when they are doing roughly the same speed, thus blocking the road for car drivers for ages?' So here's the answer: Well, if you are only a car driver you probably won't accept any of this as a justifiable answer but at least you'll have a bit more of an idea why.
These are the factors:
The speed limiter - laden goods vehicles (LGVs) over 7.5 tonnes, and some under this weight, are limited to 56mph. There is a mechanism in the engine which means that when you try and accelerate past this speed, nothing happens, the acceleration just stops. So, what is the point of trying to overtake each other then? Good question, but the fact is they are not all set in exactly the same place. Some don't go much beyond 54 some you can get up to 56.5. The supermarkets are often limited to 53 as this is supposedly optimum fuel consumption speed. If you take the A34 connecting the major transport hub of Southampton to the Midlands, many lorries are travelling the full length of it. It is 110 miles from Southampton to Northampton, a distance that would be travelled almost exclusively on dual-carriageways. A difference of 2 miles an hour would make a difference of 4 minutes on the overall journey. This may not seem like much but it brings me onto the next point -
Time pressures - lorry drivers are under pressure for time both from their transport offices and from the tachograph regulations. Most long-distance lorries are satellite tracked so their transport office knows what speed they are doing and if they are not doing the speed they could be doing, the office will want to know why. Routes are worked out according to how long it ought to take and if you are not sticking to time, connecting routes will be thrown out of schedule (this is especially the case for things like Royal Mail, DHL etc.). Routes are also worked out to make sure that your tacho breaks fall at convenient times for the deliveries. You can drive for 4.5 hours before you have to take a 45 minute break. Last week 5 minutes was the difference between me making it home on time at and me being stuck in the Midlands for 45 minutes, thus making all the post I was carrying late.
The other factors that influence my choice of whether or not to overtake and block the road for ages are:
Load differences and hills - a vehicle that is heavily laden is not going to make it up the hills as fast as one that isn't. However, while some vehicles have a feature where the exhaust brake kicks in automatically to make sure you stay at the speed the limiter is set to, many do not. This means that a vehicle that is heavily laden will actually go faster down the hills that one that isn't. Frighteningly fast in fact! (69 is the fastest I have ever got a laden lorry to go, I tried to break the speed limit for cars but I didn't quite get there. It was fun but scary and I certainly won't be doing it again!). When deciding to overtake you have to make the decision about whether overall you are going faster than the one in front or not. The A34 is hilly, so if you are overtaking downhill (or uphill, depending on which one of you is laden) you also have to decide whether you are going to make it past the lorry in front before the gradient changes again. Personally if I'm being overtaken uphill and the lorry overtaking hasn't managed to get back in by the time we are going downhill again and going the same speed, I tend to brake to let them in to get the road moving smoothly, as long as I accept that overall they are going faster than me. If I don't think they are, and think they were just being aggressive, I won't slow down because I know that sooner or later I will probably have to overtake them again. This may well end up as a scrap that blocks the road for ages.
The final factor is cruise control. Let's take the A31 coming out of Bournemouth towards Southampton through the New Forest. It has some very long steep hills and at certain times, is very busy. If you are driving a lorry up a hill on cruise control and suddenly realise there is a slow-moving vehicle ahead and brake, thereby taking the cruise control off, your speed could drop by 20mph in an instant, which is dangerous, and it can take miles on the A31 to get that speed back. If you pull out straight away and get past it, it takes seconds and everyone is happy. This is a very thinly veiled way of saying 'Oy! Car drivers! Let us pull out!'.
So anyway, these are the reasons why. Having said all that, the speed limit on dual-carriageways for LGVs over 7.5 tonnes is actually only 50 mph but I have heard from a retired policeman that there is an unwritten agreement between the police and the LGV industry that truckers won't be prosecuted for driving at the limiter on dual-carriageways. I certainly know of truckers who have been stopped doing 56 and the police have not batted an eyelid about the speed and I have never known anyone be prosecuted for it but you never know.