Sounds like the left front brake is dragging, staying in contact with the disc even though you're not on the brake pedal. Could be air in the brake line or sticky caliper pistons. I suppose another problem is that you could be leaking brake fluid onto the brakes, and when you exit the highway and apply the brakes the fluid "smokes".
Have you noticed the pedal getting softer over time? Have you seen something that looks wet around the left front brake area? Is the master cylinder reservoir low on fluid? If your answer is yes to any of these, I suggest you address fluid loss before driving again.
Mostly likely the brakes are dragging on the disc, but i would check to make sure there's no fluid loss first. If the brakes are dragging, and you do not hear metal grinding [like the brake pad material is all gone], then I don't think there'd be a problem driving to the mechanic.