YouTube - Marijuana and Driving
You can't overdose on weed, It's literally impossible. It's obviously not good for your lungs but there is absolutely no link between lung cancer and cannabis use alone. The worst that could happen is you get bronchitis if you're a heavy user.
Its funny that he said he felt like he was going slower. I dunno. I've never driven high because I still don't have a tolerance and am not used to the effects yet.
It seems like he would be feeling like he is going faster.