What does the do not enter sign mean?

This sign faces you at the mouth of a road you were about to turn into. What does it mean?

Traffic on that road travels toward you, so you must not enter it

The sign marks the exit end of a one-way street or an off ramp. A driver who goes in behind it is head-on to traffic that has no reason to expect a vehicle there, which is why the rule has no exception for entering carefully.

You are looking for a way onto a highway and this sign faces you at the mouth of a ramp. What is it telling you?

Traffic on this ramp is coming toward you, so you must not enter

The sign marks the wrong end of a road or ramp that runs one way, most often a highway exit. Entering there puts you nose to nose with traffic at full speed, which is why the sign forbids entry outright instead of asking you to yield.

This sign faces you at the entrance to a road. What must you do?

Stay out of that road entirely

This sign closes the road to you from that point, most often because the traffic on it runs toward you. There is no version of entering carefully that makes it work, because the drivers on that road are not looking for anything coming the other way.

You are looking for a way back onto the boulevard and find this sign at the mouth of a short paved road. What is it telling you?

Do not drive in here, traffic on this road comes toward you

This sign marks an entrance you must never use, most often an exit ramp or the wrong end of a one-way street. Anyone coming out of it is travelling at full speed and is not expecting to meet a vehicle head on. Find another way round rather than a quick look and a dart.

You are stopped at a corner. This sign faces you on the road to your left, and the road to your right carries no sign at all. Which turn is legal?

Right, because the road on your left brings traffic toward you

The sign speaks only about the road it guards, and it says that road is not open to you from this end. The unsigned road to your right is unaffected, so the sign narrows your choice rather than removing it.

You are driving at night in an unfamiliar town, you take a turn, and your headlights pick out this sign directly ahead. What should you do?

Stop, then turn around or back out, and find another way

If the sign is facing you, you are at the exit end of a road that runs the other way, and nothing on it expects a vehicle coming at it. Correct the mistake before you commit, because a driver coming around a bend has almost no time to react to a car in the wrong direction.