Mirror across road??

I don’t believe so. There was another movie—“Goldfinger”—made in 1964, in which James Bond (Sean Connery) was being chased at high speed at night by the bad guys. He was driving that fancy Aston Martin that had been rigged by British intelligence. He laid out an oil slick that took some of the bad guys off the road—very similar to what Mitchum does in “Thunder Road.” Later, he swerves and crashes when he sees the headlights of his own car in a mirror and thinks it is another car coming straight at him. Don’t know if you might possibly have been thinking of that.

A tad late for a response but there was another movie that I think used a similar trick ... but I can't remember now what it was exactly:
"Hot Rods to Hell " (1967) with Dana Andrews . Its an old, low budget B&W with lots of racing around. Andrews sets up something to fool the punk teen hot rodders that have been threatening he and his family, into thinking a car is coming at them head on ... at least I think so (??) - its been a long time (in dog years) since I've seen it.
Anyone know/remember if a mirror was used ?

No. You are thinking of the movie Fireball 500 with Frankie Avalon and Annette.