Driver was driving erratically, swerving, braking for no reason. I tapped my horn, thinking perhaps he/she was on the phone. He/she SLAMMED on the brakes in the middle of a 4-lane road and simply sat there, for an entire green-light cycle with no one in front of him/her. I needed to turn right or I would have swerved around like all the other drivers had to do. I was honestly afraid the driver would get out and come back to my car -all for a simple tap of my horn - because of HIS/HER erratic and dangerous driving. I photographed the license plate and vehicle.