Do you do this at round about ????

Bigphil1474

zagfles said:

Bigphil1474 said:

Car_54 said:

Bigphil1474 said:

Nope. Depending on the size of the roundabout, but for a normal road roundabout, turning right is 90 degrees, coming back on yourself is 180, turning left by going right round is 270, and going back to where you were facing in the first place is 360 degrees.
I've never done the go round the roundabout trick, i think it's rude, lazy and selfish, but each to their own.

Your driving may be good, but your maths is very dodgy.

Left is 90 degrees, ahead is 180, right is 270. Doing what the OP suggests is 450.

It's perfectly legal and I've done it. 

My maths is perfect as others have explained (from the drivers perspective) - and my O level, AO level, and A level obviously taught me summat. 

Just to carry on this pedantry for a bit for fun for us maths geeks. Your maths is wrong. (I trump your A-level with A-level Further Maths  )

I think 270 for a loop then left is almost always incorrect.

Firstly, forget mini roundabouts, as no-one is going to loop a mini roundabout to get ahead of traffic, and they rarely have 2 lane entries. OP was obviously talking about a proper roundabout.

So, if you're going by the change in direction of the car, there's 2 ways to do it.

1. Take the net change of direction, where eg turning towards the left is positive and turning right is negative. Then clearly taking the exit to your left is +90 and taking the exit to the right is -90. If you go straight on, that's 0 even if, like for most roundabouts, you have to initally turn left to enter it, even if only slightly, then go right slightly as you go round it, then left as you exit. If you do it this way, then whether you turn left immediately or loop the roundabout and then turn left, or even if you make 100 loops and turn left, it's still +90. It's the net change of direction, it's the direction you end up going that matters. So 270 is definitely wrong.
   
2. Or, take the total change of direction, ie ignoring whether left or right and adding turns to the left to turns to the right. Now if you could make the manoeuvre by only turning towards your right, then if you loop and go left it would be 270. You've turned 270 to the right. However that's very unlikely with any normal roundabout. You'll almost certainly have to turn at least a bit to left to enter it, and exit from it. So the total change of direction will be more than 270, often considerably more.
   

Therefore, 270 is almost always wrong however you look at it!

Nice to meet another maths geek, but I don't agree. Net direction change is a different thing.
The total change of direction is still 270 even if you have to jink left and then right on entry/exit i.e. your final direction after leaving the roundabout compared to the position when you entered the roundabout.  You end up 90 degrees to the left of your starting position (assuming a right angle road exiting the roundabout) after going round the roundabout and 360-90 = 270. A jink left might be say 5 degrees, but you would need to jink right to counter the first jink, which would cancel it out.
Obviously, depending on the type of roundabout, there will be minor course changes for different angles of the entry and exit roads, and the steering wheel itself will move through a variety of angles as you steer, so the accumulative angle changes could be said to be different. However, for a hypothetical standard roundabout with 4 exits (at the compass points), turning left after going right round, is a 270 change of direction. Going straight on is a 0 change in direction, even if you have to do a bit of a wiggle left and then right to enter and leave the roundabout, or vice versa. 
They didn't do A level further maths at my school, which is good, because I hated A level.

zagfles

Bigphil1474 said:

zagfles said:

Bigphil1474 said:

Car_54 said:

Bigphil1474 said:

Nope. Depending on the size of the roundabout, but for a normal road roundabout, turning right is 90 degrees, coming back on yourself is 180, turning left by going right round is 270, and going back to where you were facing in the first place is 360 degrees.
I've never done the go round the roundabout trick, i think it's rude, lazy and selfish, but each to their own.

Your driving may be good, but your maths is very dodgy.

Left is 90 degrees, ahead is 180, right is 270. Doing what the OP suggests is 450.

It's perfectly legal and I've done it. 

My maths is perfect as others have explained (from the drivers perspective) - and my O level, AO level, and A level obviously taught me summat. 

Just to carry on this pedantry for a bit for fun for us maths geeks. Your maths is wrong. (I trump your A-level with A-level Further Maths  )

I think 270 for a loop then left is almost always incorrect.

Firstly, forget mini roundabouts, as no-one is going to loop a mini roundabout to get ahead of traffic, and they rarely have 2 lane entries. OP was obviously talking about a proper roundabout.

So, if you're going by the change in direction of the car, there's 2 ways to do it.

1. Take the net change of direction, where eg turning towards the left is positive and turning right is negative. Then clearly taking the exit to your left is +90 and taking the exit to the right is -90. If you go straight on, that's 0 even if, like for most roundabouts, you have to initally turn left to enter it, even if only slightly, then go right slightly as you go round it, then left as you exit. If you do it this way, then whether you turn left immediately or loop the roundabout and then turn left, or even if you make 100 loops and turn left, it's still +90. It's the net change of direction, it's the direction you end up going that matters. So 270 is definitely wrong.
   
2. Or, take the total change of direction, ie ignoring whether left or right and adding turns to the left to turns to the right. Now if you could make the manoeuvre by only turning towards your right, then if you loop and go left it would be 270. You've turned 270 to the right. However that's very unlikely with any normal roundabout. You'll almost certainly have to turn at least a bit to left to enter it, and exit from it. So the total change of direction will be more than 270, often considerably more.
   

Therefore, 270 is almost always wrong however you look at it!

Nice to meet another maths geek, but I don't agree. Net direction change is a different thing.
The total change of direction is still 270 even if you have to jink left and then right on entry/exit i.e. your final direction after leaving the roundabout compared to the position when you entered the roundabout.  You end up 90 degrees to the left of your starting position (assuming a right angle road exiting the roundabout) after going round the roundabout and 360-90 = 270. A jink left might be say 5 degrees, but you would need to jink right to counter the first jink, which would cancel it out.
Obviously, depending on the type of roundabout, there will be minor course changes for different angles of the entry and exit roads, and the steering wheel itself will move through a variety of angles as you steer, so the accumulative angle changes could be said to be different. However, for a hypothetical standard roundabout with 4 exits (at the compass points), turning left after going right round, is a 270 change of direction. Going straight on is a 0 change in direction, even if you have to do a bit of a wiggle left and then right to enter and leave the roundabout, or vice versa. 
They didn't do A level further maths at my school, which is good, because I hated A level.

Ah yes, I see what you mean, so that is basically net change of direction but by reference to the current position of the car rather than initial position, as I based 1. on.

So if the roundabout was a perfect circle and ignoring stuff like multiple lanes, if you entered and turn left normally that would consist of a 90 degree left turn as you enter the roundabout, followed by a gradual 90 degree turn right round the circle, followed by a 90 degree left turn as you exit, net result is 90 degrees left (180 left minus 90 right).

If you loop the roundabout and go left, then that would be a 90 degree left turn on entry, followed by 450 degrees gradual right turn round the full circle and a quarter, followed by a 90 degree left turn to exit. So net would 270 right (450 right minus 180 left).

Obviously real roundabouts aren't like that and the entry and exits would be less than 90 degrees but the net result would be the same.

So the difference between the 270/450 schools of thought is that the 270 includes the entry/exit manouvers whereas the 450 doesn't, it's approx change of direction once on the roundabout. Or more accurately, to account for multiple lanes and different shaped roundabouts, the change in your direction from the POV of an observer in the middle of the roundabout.