I was riding with my sister who is driving a 1992 Honda Accord. At around 55 MPH or higher, the car vibrates extensively, only being mostly alleviated when the foot is depressing the accelerator. What might be the cause of this vibration?
My guess is a bad tyre or a suspension problem, likely on the front end.
Seeing as the car is 18 years old, I strongly urge you (or her) to get it to a reputable chassis shop ASAP. And I suggest having it towed.
There’s a number of possibilities, most of them not pretty, and it’s impossible to see under the car from here. This could easily be a worn out suspension compont about to fall off.
If the problem pretty much goes away when the accelerator is pressed, it is more likely CV joints than tires.