Posted by Have you ever wondered why most motorway roundabouts are above, rather than below the motorway?
Well, the answer is simple, because it elevates the beginning of the joining sliproads and the end of the exit sliproads. This gives you the help of gravity to achieve your aim.
Sliproads should more correctly be called acceleration lanes, or deceleration lanes because that is, in fact, their primary purpose. (I’ve actually just been told by Dave Richards who knows his stuff, that they are actually called GSJ’s or Grade Separated Junctions. I stand corrected Dave) They are there to give you a chance to accelerate your vehicle to the same speed as the traffic already on the motorway carriageway or to leave the motorway at the same speed as traffic remaining on the motorway. That way, you can simply slot into the gaps that have hopefully been left between those vehicles. We know this isn’t always the case, but with a little more thought, understanding and care, it should be.
If the sliproad onto a motorway runs uphill, then the vehicles joining, particularly lorries, are likely to be travelling significantly slower than if the sliproad runs downhill to join the carriageway. This is something that as a driver, I try to bear in mind if I’m passing a junction. If I pass an exit where I can see the sliproad runs downhill off the motorway, then I prepare to move into lane 2 before I reach the joining sliproad that will be running uphill. I don’t move for the sake of it, but if I see slower traffic joining, I am already in a position to move into lane 2 to allow the slower moving traffic to join lane 1 seamlessly.
How many times have you had to brake because all the traffic bunches up as it passes the joining sliproad? How often have wondered why the traffic suddenly speeds up again a few hundred metres after a junction? Well, it’s because drivers on the main carriageway were not prepared, and not usually because the joining drivers were barging in, although that does happen too.
Incidentally, I live near a motorway junction that has the best option, joining downhill, and exiting uphill. There are rarely jams in either case. The next junction down the motorway is unusual in that it has both. On the Southbound carriageway, it has a downhill exit and downhill joining. However, on the Northbound carriageway, it has uphill exit and uphill joining. This causes no end of traffic jams around the junction, even when the rest of the motorway is flowing freely. All of this because most drivers are ill-prepared for what’s about to happen, and find themselves suddenly slowing down and trying to change lanes.
So, what should you do on a sliproad? Well, firstly, get into the habit of calling them acceleration or deceleration lanes. That way you mentally prepare for what you need to do on them. If the acceleration lane slopes downhill onto the motorway, then you have gravity on your side and acceleration to the correct speed should be fairly easy. Remember you are aiming to match the speed of vehicles in lane 1, regardless if they are doing 50, 60 or 70 miles an hour. Just glance right and see if you are travelling at approximately the same speed.
If you are exiting the motorway, then it’s a bit more complicated. The deceleration lane is usually long enough, and straight enough to allow you to leave the motorway at 70 mph and have time to decelerate to a stop before you reach the junction ahead. In practice, there are things to take into account, such as; are there other slower moving vehicles ahead, or is there stationary traffic on the sliproad? Is the road surface wet, or does the sliproad bend significantly? Does the sliproad slope up or downhill? If it slopes uphill, again you have gravity on your side.
So, once again, preparation is the key. Be aware and leave a sensible gap to the vehicle in front. Watch for brake lights a good distance ahead, and if you are not familiar with this particular junction, leave even more time to assess the conditions.