Agreed. Actually, I don't think the shooter is using the highways to any great degree at this point as it would be far too risky. Anything other than just a short hop on hop off to get away from the immediate area right after the shooting would be inviting capture. He knows that police are going to set up roadblocks over a large area so why take a chance on not getting far enough away to elude them. Not to mention the possibility of getting caught in a traffic jam and running smack dab into a roadblock. This guy or guys have this way too well thought out, IMO.
In line with a comment made some way up the thread, my gut tells me the shooter is staying within the area. Even if the shot is heard right away and reported quickly there's going to be a few minute delay before LE reactions come into play. If the escape route and ultimate escape destination is well scoped he could be out sufficiently away from the scene and out of the vehicle if he's using one (and as was suggested maybe in a coffee shop or restaurant) before the news even breaks.
If you know where you're going and have a reasonable assurance that traffic gremlins won't come into play its amazing how much ground could be covered in 3-5 minutes.
Bottom line...I think the shooter stays close to the event. He's not getting on the highway or if he is he isn't staying on it for more than a minute or so.
I don't think you can ever realistically expect roadblocks to go up faster than that.