Most of the time, there's an empty lane as a separator. When it is used, they don't allow motorbikes in it just in case, but it's really no worse than a two-lane undivided road.
It's also not necessary for you to use that lane if you don't want to -- it doesn't go anywhere special :).
As a correction, as this road appears on the BBC's Motorway Cops regularly, and there was an incident featured in this weeks' episode (1): There is always an empty lane, which shifts depending on the time of day. Accidents are apparently very rare, and the speed limit is 50 mph, compared to 70 on a regular motorway and 60 on a 'normal' road.
It's also not necessary for you to use that lane if you don't want to -- it doesn't go anywhere special :).