Thanks you for your answers and for reminding me of the importance of capitalization. I agree with the order: could
/ would/ crossed. I would like to push the debate a little further especially that in an exam correction situation we have to accept all grammatically correct answers even less probable ones.
If you consider the following context:
The policeman stopped the traffic but the blind man has not started crossing the street yet or is taking time to do so. Would the present or can be accepted? (I mean is there any possible context that allows the use of can or the s.present?)
First, a few points.
1) To me, "so" doesn't always mean "so that".
"The policeman stopped the traffic so [that] the man could cross the road." (The reason that the policeman stopped the traffic was in order to allow the man to cross the road.)
"The policeman stopped the traffic, so the man crossed the road." (The policeman stopped the traffic for reasons best known to him, and the man took advantage of this by crossing the road.) I don't think "so that" belongs with that meaning, so I would reject "crossed".
"The policeman stopped the traffic so [that] the man would cross the road." (The policeman wanted the man to cross the road, and he decided that stopping the traffic would make that happen.) This is unlikely.
In my opinion, "could cross" is the only likely answer.
2) "...
especially that in an exam correction situation we have to accept all grammatically correct answers even less probable ones."
I disagree with this. There are many weird senses/contexts in which a sentence
could be made grammatical. It is merely sufficient to make a note on the exam that only the most likely answer is correct. Of course, you
do have to know which is the most likely answer, but that's not an added problem because, with your idea, you have to know all the weird grammatically correct possibilities.
To your question: "can" is not right in that sentence. The main clause is in the past tense. He stopped the traffic.
By "s.present", are you asking whether, "The policeman stopped the traffic so [that] the man
crosses the road"? That's wrong too for the same reason.