What percentage would you guess of first plays in a game is a handoff up the middle? I'd opine a running play is called 80% of the time in P4 CFB. Up the middle might be lower of course, maybe someone runs a sweep of some sort. I'm calling "up the middle" to include any play meant to go behind any part of the OL, traps, etc.
If you get 7-8 yards, you run it again. It can be frustrating to see running plays run gaining 5-6-7 yards a whack and then on 2nd of 4 they call a pass play, incomplete, now it's 3rd and 4 ...
Straight gap run plays are rare nowadays. Those are the plays when the RB knows he's going between the Center and Guard, or Guard and Tackle. When that would happen, the OL more or less knew who to block. The RB was heading for one place, and that place better be cleared.
Trouble with that was, the defense wouldn't play along. LBs would appear, STAR players or extra Safeties would creep into the box. The gap would get flooded by these defenders while leaving other lanes empty.
Today, you'll see "inside zone" or "outside zone" runs. In these runs, the OL more or less blocks the man in front of them. Sometimes, they'll block left or right and try to "wash" the defenders down. At any rate, the RB is tasked mostly with finding the first gap freed by the OL. This forces the defense to play all gaps, or at least hold back the LB or secondary until the runner commits. An OL player that beat his man is encouraged to then work to those defenders.
The zone runs are aided by the shotgun and offset stance of the QB and RB. It lets these players view the developments before committing. With RPO schemes, the QB has the option to pull the ball to either run it himself or set up a quick pass (he reads the actions of like 2 key defenders to make his decision).
The downside is that the play can be slow developing. Any breakdown could have the RB immediately tackled for a 5 yard loss.
