Several years ago, another pickup truck pulled up beside mine on the highway. There were 2 men (early, mid 20s) in the truck, and it simply stayed beside me. The passenger was leaning out of his window, and looking at me intently. I slowed down, hoping they would just pass me, but they slowed down too. So I sped up, and they sped up and still stayed beside me. So at about 70 MPH, I just slammmed on my brakes. They couldn't react quickly enough, and they pulled ahead for brief period. But then they pulled into the right lane in front of me. I switched to left lane to try to pass, they pulled left and blocked me. Regardless of which lane I moved to, they blocked me, and began slowing down. At this point it seemed certain they were trying to force me to a stop. And the passenger alternately leaned out the window to look back at me, then lean back in and talk to the driver.
I was carrying my Taurus Model 66 .357 in the glove compartment, and reached over and got it out and layed it on the seat. When I looked back up the passenger had pulled his head back into the truck, and was gesturing excitedly to the driver, and they suddenly stepped on the gas and started pulling away. I was puzzled by this for a few seconds, then I realized that when I pulled the gun out of the glove compartment, I had (inadvertantly) lifted it above the dash, and made it plainly visible through the windsheild. They continued to pull away from me, and exited at the next exit.
I will never be sure exactly what their intent was, but feel sure it was a genuine threat. And as is often the case, simply demonstrating that I was armed was sufficient to end the threat.