Approaching a Roundabout
When approaching a roundabout, you should use Mirror - Signal - Manoeuvre at all times and continually adjust your speed and position to fit in with traffic conditions. Before you get onto a roundabout you should make sure that you are fully aware of the situation around you - including where you are entering the roundabout and which exit you should be leaving it from. On approach, you should be able to see a road sign that will indicate the shape of the roundabout and how many exits there are in total.
Safely Navigating a Roundabout
If you get your speed and gear right and the way is clear, on many occasions you will be able to merge with the moving traffic without stopping. Once you get onto the roundabout, keep a reasonable speed otherwise you may find that other drivers are confused by your actions and may end up overtaking you on the inside, which could cause problems for you and other roundabout users.
Always keep an eye on your mirrors but most importantly watch the car in front of you. Many accidents on roundabouts occur as drivers rear-end the car in front of them because they are watching the traffic on the roundabout not the traffic waiting to get on it. Make sure the vehicle has actually moved away before you start to move forward.
It's also important to be aware that many people don't understand roundabouts, or how to signal properly on them, so what they are telling you with their indicators might not be what they are actually about to do.
The following advice assumes a roundabout has 3 exits and is a handy guide to follow:
To Turn Left (leaving at the first exit)
Signals left, and approach the roundabout in the left-hand lane.
Keep to the left of the roundabout and maintain the left signal to leave
Choose the appropriate lane on approach to and on the roundabout. If you can't see the markings on the road then it is usually the safest option to keep in the left lane.
Stay in this lane until you are getting ready to exit the roundabout
Indicate left after you have passed the exit before the one you want.
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To Turn Right (leaving at the third exit)
Indicate right and approach the roundabout in the right-hand lane
Keep to the right-hand lane on the roundabout until you need to change lanes to exit the roundabout
Signal left after you have passed the exit before the one that you want.

In all cases, you should make sure to watch out for:
Pedestrians who may be crossing the approach to the roundabout or exit roads.
Traffic crossing in front of you - most importantly vehicles that are going to leave at the next exit
Traffic straddling lanes or positioned incorrectly ,traffic might pass you on either side
Cyclists and horse riders (who must stay in the left hand lane but signal right if they intend to go around the roundabout
Long vehicles - they may have to move across several lanes, so be careful and watch for their signals.
Blockers
In the diagrams below the yellow car could use the cars coming around the roundabout to block the cars to the right to make progress
If the yellow car uses the blue cars as blockers it will need to be timed better as the blue car could be going full circle . so you would probably need two cars coming around as blockers

Explanation of some types of signs on approach to roundabouts
On a standard four junction roundabout usually you would approach in the left lane to take exit A or B and the right lane to take exit C or go full circle
On this roundabout sign exit C is a minor local road so exit A would be the left lane and exit B would usually be the right lane as they are the two primary routes
On a roundabout similar to this , exit A is a minor route , so to take exit B you would approach the same as following the road ahead with no signal and then signal left to leave the roundabout after you pass exit A
On this roundabout exit's A and C are minor roads so to take exit B you would usually use the left lane
On this roundabout because there is no exit on the right , so you would usually use the left lane for turning left and the right lane for following the road ahead
This sign means you need to go full circle at the next roundabout to get to a destination
Sometimes you get two roundabouts close together , like at the end of a slip road after leaving a motorway or dual carriageway , treat each roundabout separately ,
The diagram below shows two no entry signs , they would be the slip roads off the motorway or dual carriageway
Exit B and C are the primary routes off the first roundabout , so the left position on approach would be for exit A & B , to follow the road ahead towards the second roundabout , exit C you would usually be in the right lane on approach
At the second roundabout exit D is a no through road and exit E is a minor or local road , exit F is the only primary route , so exit D & E would be the right hand lane on approach and exit F would be the right turn position

The diagram below shows you the route to take for junction F
For more information on road signs click