A slip road is used to accelerate your speed in such a way that you can time your entry to the carriageway to coincide with an appropriate gap in the traffic from your right. The objective is to equal your speed to the speed of the traffic on the carriageway such that the needed gap would be minimal.
Slip roads were developed to help reduce the dangers in joining on to the carriageway. A slip road is the commonly used term to describe a deceleration or acceleration lane that helps maintain the flow of traffic on the dual carriageway by giving a much safer way to leave or join the fast flowing traffic. It is the only allowed method in keeping safe on motorways.
The gap needed will have to be much larger if you are travelling at a slower speed. Slip roads unfortunately come in various lengths, thus it is not often possible to equal your speed to the speed of the traffic you intend to join with. So, even before you start the manoeuvre to join a carriageway, you must estimate the length of the slip road in determining what gaps there are in the traffic from your right and what speed you can attain. Adjust your speed such that you can reach the speed you need once you can see a reasonable gap coming towards you in the mirrors to safely merge as the gap appears to your right at the same time.
Just before joining the carriageway, you then need to check your mirrors again and possibly glance into your right hand blind spot. Therefore, you must use at least twice your hazard drill with close observation through the mirrors to complete your manoeuvre.
Then, watch out for the end of dual carriageway sign. After you see the sign don’t overtake the vehicle you are following in front of you, or else you might be short of road or even worst, you will find yourself facing a coming vehicle head on from the opposite direction.
You must also bear in mind that some slip roads are quite short, so the gap in the traffic that you need to join in needs to be much bigger to ensure your safety. You may also stop at the start of the slip road if the carriageway was very busy before you start the manoeuvre.
When a slip road is available or not too busy whilst you leave a dual carriageway, you should try to avoid reducing your speed as much as possible until you have entered the slip road. If the slip road is too short and it is not safe for you reduce speed, then you will need to reduce speed before you enter the slip road. In such case, you use your hazard drill twice effectively, that is when you reduce speed just before entering the slip road and another one is when you begin to enter the slip road.
It is worth bearing in mind that on carriageways vehicles are allowed to run at the maximum speed of 70 miles per hour, so you can already imagine how difficult judging potential gaps between the speeding vehicles that you need when you join in from a slip road to turn right onto a carriageway. So, you need to be extra careful always.