The motorway is the safest place you can be (other than at home in the garden sunbathing!).
All the traffic is going the same way, at give or take the same speed, and you can only enter in 1 direction (not the band!).
By law, you have to stick to the slow lane unless overtaking, and you can be pulled for middle lane hogging, even though as Scotty says, it's the dirtiest lane usually. Even so, you're going in a straight line so shouldn't have any probs.
If you are in heavy traffic and filtering, THE SAFEST PLACE TO FILTER IS BETWEEN THE MIDDLE AND FAST LANES!!!! Just keep an eye on your mirrors, remember lifesavers, and watch thehead of the driver of the car in front to see where he's looking, and therefore likely to go. Also, it's a good way to spot undercovers, as they quite often still wear uniform in unmarked cars...
