I was in the left lane, accelerating to freeway traffic speed on the Horizon/11 onramp. This ramp merges 3 lanes of traffic into one with a fairly short unprotected merge at the end. The driver of this vehicle, a white or hispanic man, bearded and bald, pulls up to my right and matches my speed on the on-ramp, despite being behind me the entire time. He stays in that line, side-by-side with me, while the lanes merge into one. Had I not rapidly applied the breaks, he would have forced me either into traffic that was moving slightly faster than we were or would have hit my car on the passenger side. He would have caused an accident had I not been defensively driving and paying attention. I was still in a dangerous spot as I had just nearly come to a stop on the ramp with vehicles around and behind me. I had to downshift from 5th to 2nd gear and nearly floor it to safely merge. I accelerated to around 75 MPH, the flow of traffic, and saw his vehicle quickly pulling away from mine. I assume he was probably doing 85-90. This type of behavior WILL cause an accident eventually and may get himself/his passenger/someone else killed or injured, not to mention property damage. Learn to drive, be considerate, and have some patience before its too late.