Some large lorries take a considerable time to get up to full speed, and loosing speed, and needing to start the build-up again, can sometimes add as much as twenty minutes to a journey time. Lorry drivers are usually on a delivery schedule, and will always try to get ahead of it if they can - the earlier they deliver, the earlier they can finish work, so they will take an opportunity to maintain speed if they can. It's not just being awkward, this is the genuine reason why they do this.