You need to talk to him, calmly and without judgment, ahead of the trip. When you're in the middle of driving and bring it up, it's too high-tension and he's already feeling aggressive.
Use lots of "I" statements, don't say "you do this." Something like "I get really nervous when we're close to other cars in traffic or going above the speed limit, and it's hard for me to relax and enjoy our trip."
My husband personally responds well to something like "It would really mean a lot to me if you'd be a bit more conservative, just to indulge me. You're a great driver and I appreciate that you've always gotten us there safely, and I would enjoy the trip even more if I didn't feel nervous in the car." It irritates me sometimes to have to protect his ego and be careful when phrasing requests (my natural style is much more direct/demanding), but it is what works and puts his brain into "take care of my wife" instead of "argue with my wife" mode. I'd rather be happy than right, so I do it.