What would you do?

[Slink_Pink]

There is a traffic light controlled junction, two approach lanes at which you can proceed ahead (either lane), turn right (lane 2) or a cyclist may turn left. You are approaching traffic lights during green phase, second car back from the stop line in lane 1, intending to proceed straight ahead. Traffic is queuing in lane 2 to turn right. The car immediately in front of you signals left, obviously oblivious to the no-left-turn restriction in place.

Should you:
(a) attempt to warn the driver in front of their error, perhaps via horn + polite gesticulation
or
(b) sit back and watch them get into a mess?

Just happened in front of me today as I was driving. As it was, I chose (by the act of doing nothing) option b. Slightly entertaining for me, but perhaps not the approach of a responsible road user. After turning left, the car began trying to reverse back into the junction, then went forward into the cycle lane, then gave up and asked a passing builder for help. Once the lights had cycled around again, there was plenty of space for me to drive ahead as planned.

Discuss!

[dombooth]

I would say just let them get on with it tbh.

Most people see the horn as a way of saying 'f*** off'.

Dom

[Ancient]

"I am not a policeman"
Tempting though it is to try to correct other road user's behaviour, since I am not a policeman I have no authority to do so, have no tools to do so, am concerned with remaining safe myself. If I were to see a situation developing where my sounding my horn might prevent someone being injured, then I would do so; but the horn is a warning of my presence, not that of others so in the situation you describe could make matters worse (by distracting the driver or others and so causing them to collide).
Option b for me.

[driverpete]

I'm with Dom and Ancient. In this instance sounding the horn or flashing lights are more likely to confuse the issue added to which the driver in front may be perfectly aware of what he's doing and made what to him was an appropriate choice.

[PeterE]

Probably best in all circumstances to turn the other cheek, but there is a difference between someone making a prohibited turn into a road they can actually drive down, and trying to make a turn that isn't physically possible, which sounds like what happened here.

[apple tango]

I would let them get on with it.

I don't see what any signal from us as a driver behind them would achieve, other than negative reactions or confusion. If we were to rebuke everyone who we saw break the law we'd never be off the horn!

[Slink_Pink]

Well that makes me feel better! From my perspective it appear that the driver was unfamiliar with this particular junction. As PeterE said, it was physically impossible to make that turn in a car - there was a single lane to enter the junction with a contraflow exit cycle lane separated by kerbstone and I think bollards, sadly it can't be seen on any of the online maps. The local signage is slightly misleading though as it has not changed since the restriction of this part of road.