A circular red sign with a white horizontal bar means:

A circular red sign with a horizontal white bar (or a 'DO NOT ENTER' sign) means do not enter the roadway in the direction you are facing — usually because the road is one-way in the opposite direction, or it is an exit ramp. Entering a road marked Do Not Enter creates a wrong-way driving situation, which is among the most dangerous types of crashes (often head-on at high speed). If you find yourself going the wrong way, pull off the road as quickly as possible, turn around, and rejoin traffic going the right direction. Highway exit ramps are commonly marked with Do Not Enter signs at their downstream ends to prevent wrong-way entry.