No one's saying that the Secret Service did a stellar job that day, although it certainly could have been worse and they weren't to blame for everything going wrong.
We all see your point crystal clear. What we're questioning (at least I am) is if you're getting the protocol right. All the evidence from that day suggest that reversing position wouldn't be correct protocol. But, for the sake of argument, let's say it was. Would it be safe to assume that there would be a "Plan B" in place? Plan B being, "you know, if we have a motorcade behind us, perhaps we should hit the gas to the nearest hospital instead of running into to them, not knowing when or where, exactly, the shooter has the best opportunity to shoot. After all, we can rest assured that we're out of the target area once we get passed the underpass."
Two things:
1.) We know where the shots we're fired from after the fact. The Secret Service did not know at the time of the shootings where they were coming from. In fact, I'd say this is quite clearly the most controversial aspect of the entire assassination, yes?
2.) There were NO shots fire after the limousine sped off. I'd say that's pretty clear for a couple of reasons: The killer already had his head shot and there weren't any more opportunities as the limousine was out of sight. Again, this is pretty obvious if you've looked out the building from Oswald's window. There's a video