The only allowable collisions are incidental ones. There is no allowance in the rules to intentionally run over players.
Intent is considered malicious and the result is often ejection.
Just as the defense is not allowed to block the baseline without having the ball, runners should step to the side and not create contact.
Incidental contact is when both players are moving to the same spot as the ball arrives and the defense attempts make a catch and tag.
Just to be general.
