Major Issues

So I have a huge problem.  My 2 year old son is fast as lightening and likes to go outside with no one knowing.  Today my husband went to the bathroom and forgot to lock the door.  I am upset but I don't blame him because it is very easy to leave it unlocked.  But my son goes out the front door.  By the time my husband realized he was gone my son was near a very busy road.  Luckily a passerby stopped and kept him from getting into the road until my husband could reach him.  I am so scared that something bad is going to happen and he will be hurt or even killed.  We are planning to move in about a week so I know that we can baby proof this house better than the one we are currently living in, but it is so scary to think what might have happened.  The new house is going to have a security system that will sound when the doors are open so I hope that will help alleviate the problem in the future, but we still have about a week or so of having to watch him like a hawk.