Driver's side front brake locked up

Several things can cause this.

• The caliper is mechanically stuck, requires replacement not much you can do on the side of the road.
• The brake hose for that side has failed and now is working like a one way valve, allowing fluid to apply the brakes but not allowing the fluid to release and return to the master cylinder. You need a new brake hose and even thought that is a fairly simple repair it's not something I would suggest doing on the side of the highway. Cracking open the bleeder screw on the caliper may release the pressure and get you going again, however you have a very good possibility that it will do the same thing again almost immediately.
• Problem with a valve in the ABS system. Need a shop, not really a DIY project.

Bottom line call a tow truck. Have it towed home if you think you can do it, but I would recommend towing it to a shop.

There are a couple of possibilities for the brake seizing. The caliper piston may be seizing in its bore, and will not retract. This will mean a new or service replacement caliper. A second, more usually found cause, is the slider pins are seizing. Ease the caliper off of the pins and clean up the pins or renew them. As for the bore the pins sits in, tear a strip of fine emery cloth to the width of the length of the pin. Wrap the emery cloth-strip around a suitably size split-pin and put it into a drill. You can operate the drill and with the emery/pin setup entered into the pin bore hole clean out the bore of any rust. Lubricate the pins with a copper grease on re-assembly.