Hi again, I'm coming with some more questions

I've been doing some english conditional sentences ( type 2) exercises where I had to rewrite the sentences using an if construction. And I there were two sentences with the same mistake:
a) People drive very fast, That's why there are so many accidents. I've written:
If people drove SLOWER, there wouldn't be so many ...
B) English people speak very quickly. Perhaps that's why I can't understand them. I've written:
If English people spoke SLOWER , I might be able....
As I expect you understand, the problem is with the word "slower" I've been searching the internet and some people say that both phrases:slower, more slowly are acceptable, but the answers are with a phrase "more slowly". So pleae, could somebody explain why do we need more slowly?
And by the way, do we need commas after the if clause which is in front of the sentence? In one book I've found a rule that says we need to put the comma, but in the book with those exercises answers ( a) b) and others) are without commas. So how should I write?
Thank you
