Questions about sleep restriction

Hello,
I want to get on CBT-i, and I have two worries about sleep restriction.
1. I have to drive two miles to and from work every day (math professor). Does this mean that sleep restriction is not a good idea for me? I have seen people claim that those who work heavy machinery should not undergo CBT-i as the sleep restriction is dangerous for them.
2. I notice that sometimes, I feel overwhelmingly sleepy but successfully fight it off if I feel it's too early to go to bed, only to not get sleepy again and have a bad night. If restriction says I can only spend (say) 6 hours in bed, could it be better to simply go bed whenever I feel sleepy and set a timer for six hours, rather than have a fixed to-bed and out-of-bed time? Is this consistent with CBT-i?

If you can only answer one of these, that's okay.