The Warren Commission didn't use any testimony from eye witnesses who didn't say or couldn't be coerced into saying what they wanted to hear. There are countless people who were interviewed that day who said they smelled gun powder near the grassy knoll, there were some homeless fellas on or near the overpass who said the same thing. There were people interviewed on the other side of Elm that didn't mention the school book depository at all, they talked about the grassy knoll.
Also, as
@outofyourmind said, why not take the shot as they are nearing elm and slowing down to an almost stop to turn, wouldn't have to worry about that pesky tree (some believe the tree is why Oswald missed the first shot) being in the way.