Originally Posted by djseto
Also, while you don't have instructors in you for most of the activities, you get instant feedback via radio from them and it absolutely makes a difference.
Depending on the HPDE organization, some require an instructor in the car with you for beginners, until the instructor signs you off for solo driving. I run with NASA, and if your HPDE1 (beginner), you always have an instructor. HPDE2+ is optional to request one.
Excellent to have, even if you have experience, but are going to a new track.