I’m sorry you were in a serious car accident. Its current emotional impact on you is a very common repercussion.. After the physical bruising and fractures mend the mental scars or PTSD emerge and can take quite a bit of time (and therapy) to work out.
The fact that your boyfriend isn’t able to consider this on his own and secondly doesn’t respond appropriately once you’ve communicated your duress in the car is flat out horrible.
This is a clear situation in which your boyfriend either doesn’t have the intellect or emotional capacity to behave respectfully toward you. You are asking him to stop. Period. Whether that means stop tickling you, stop having sex with you or stop driving fast – he’s not getting it. What’s more, the business of “making” a person feel or do something is not a business you want to be employed in - especially regarding an issue as black and white as this.
If he can’t adjust his driving style in your company then you seriously should consider pulling this relationship to the curb and exiting. There’s enough unforeseen crap on the road of life for us all to deal with, it’s up to us to dodge the things we can.