In the movie Ready Player One, after Parzival finds out to go backwards in the Level 1 game, why can't he make a U-Turn and drive the car in forward direction. Why he has to risk driving the car backwards?
I think the door wouldn't open if he tried to drive the car foward.
If you remember, there is a scene where:
After an argument with Art3mis, Wade visits a scene in Halliday's Journals where James Halliday argues with Ogden Morrow over the OASIS and Halliday's hatred for making rules. The scene contains the clue to beating the race: Halliday states that he wishes they could go backwards for once really fast. Source
The going backwards part is really important, and the door would know if the driver was driving backwards or foward.
And as @BrettFromLA pointed out, maybe the "drive backwards" rule was applied to the whole underground track, if he tried turning around, he would lose the race. I don't even think there was enough room for him to turn around inside the track:
James Halliday's clue was this: "Why can't we go backwards, for once? Backwards, really fast. Fast as we can. (laughs) Really put the pedal to the metal, you know?"
If Parzival took that literally, that meant that he had to go backwards, as fast as he could, for the entire race. Parzival may have assumed that if he spun his car around on the underground track, he wouldn't be following Halliday's instructions anymore and would lose the race. And watching the scene, there was really no reason to risk turning around; Parzival was driving backwards at full speed without any problems.