This is the horrifying moment an out-of-control car goes off a main highway in San Francisco.

The dashcam footage, captured on January 26, shows the vehicle plunging off the road at high speed and crashing into a traffic sign.

The witness writes: ''It appeared that the driver was swaying side to side in the driver's seat prior to the crash, which makes me guess he was under the influence of some substance.

''Prior to the crash you can see the car make a weird and illegal lane change.''

The crash happens towards the end of the clip. This video was filmed with my dashcam in a Lincoln Towncar on the way to the airport. The audio is disabled in the dashcam settings for privacy reasons, so there's no audio in this clip. The camera is always automatically recording when the car is running, so it was able to capture this clip. It appeared that the driver was swaying side to side in the driver's seat prior to the crash, which makes me guess he was under the influence of some substance, but I cannot know for sure. Prior to the crash you can see the car make a weird and illegal lane change. This happened Fri Jan 26, 2018 around 3:30pm PST on the 101 south freeway right near the SFO exit.