I suppose that 1 is "more correct", as substituting AS for So makes it a reasonable, though unnecessarily wordy, English sentence. The buses should go at such a speed AS would make it possible to prevent pedestrian accidents. I would just write: The buses should go at a speed which prevents pedestrian accidents.

The buses should travel at a speed that produces the smallest number of pedestrian accidents. "To prevent pedestrian injuries, buses should travel at a reasonable and safe speed."

Both sentences need a little work. I can't use the word "such" in the sentence without it sounding weird to me. I would word it like this: "The buses should go at speed that would prevent pedestrian accidents." You could use "which" instead of "that" in the sentence but that's how I would word it.

The buses should go at such a speed which would prevent pedestrian accidents.