A “Left Turn Only” sign means that the lane under that sign is dedicated exclusively to left turns. If you’re in that lane, you must turn left at the upcoming intersection or junction; you cannot go straight or turn right from that lane.
Where you commonly see it:
Often hanging above or posted at a left-turn lane at an intersection. For example, a sign might be above the leftmost lane saying “Left Turn Only,” indicating that lane doesn’t continue straight through the intersection.
Painted arrows on the road might also accompany it (like a left-turn arrow painted on the lane).
Key implications:
If you accidentally find yourself in a Left Turn Only lane and you intended to go straight, you’ll have to turn left. It’s illegal (and dangerous) to try to go straight from that lane because other drivers won’t expect it (and the straight lanes likely go straight at the same time you have a green arrow or such).
Conversely, if you want to turn left, make sure you’re in a lane that allows it. This sign guides you to the correct lane for turning.
Traffic rules enforce that movement: you could be ticketed if you disobey the lane-use sign (like going straight in a left-turn-only lane is effectively running a traffic control device).
So, basically, “Left Turn Only” means that lane must turn left. No other maneuver is allowed from that lane. It’s often put in place to manage traffic flow, so that a left-turn lane doesn’t also hold up through-traffic, etc.
