A flat tire on the freeway or any other busy highway can be dangerous.
There are mainly two kinds of flat tires, either the tire is very low on air or the tire is already flat and beginning to fall apart.
In the first condition (tire very low on air) you have a chance to get off the freeway to a safe place where you can change the tire or wait for service,
In the second case (flat and falling apart) it is best not to worry about saving the tire, your main objective is to get to the freeway off-ramp or to a safe place, the car will have more rolling resistance but it will continue moving on the flat tire, proceed very slowly and use caution, most of the time when a tire is run very low on air the inner layers of the tire will breakdown rendering the tire useless anyway. Don't get hung up on the idea that you need to save the tire or rim. Far too many times people try to save a sixty five dollar tire and end up getting seriously injured or worse in the process.
If a safe place cannot be found and the car will not move, walk as far away from traffic as possible to a more protected area.