I see why the first lady hopped into the limo! To prevent WWIII: If the limo does get attacked, then it becomes less plausible to accuse Pres. Logan of leaking the info to the terrorists.

or i was thinking it would force the president to try to stop the attack

they probably didn't know which plot line they were going to follow at the time of writing the script - the episodes are only written and filmed a couple of weeks before they air.