Why does time slow down when you feel scared but not on theme parks?

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I was thinking back on all the times I got a fright, from getting hit by a bus in the arm, to nearly falling, to falls, to nearly getting hit by a car, from people giving me a fright. I remember this odd feeling like that moment slowed down, and that made the fright last 10 times longer, however, I love theme parks, and despite being terrified on them, I find the rollercoasters have some odd exception. Now of all the places, I'd love that to slow down, but it goes so fast. Why is this? Is it because we know it is going to happen, so despite being totally scared, our subconscious awareness shuts down that reaction? I wish it would slow down, I enjoy theme parks so much, but just when I want it to slow down, bang its gone so fast, faster than usual, time actually seems to speed up. It's like the thing they say that time speeds up when you're having fun takes over the time slowing down when scared... Anybody have an oppinion? Any opinions are welcome including any scientific explanations.