Hehe, I had my fair share of lates in the book... With shift work it is sometimes hard to remember when you are actually supposed to be on or off especially if they change the shifts on you all of a sudden (had that happen). Plus traffic was always a nightmare. (I was fulltime and had to fight my way through M's&D's school run EVERYDAY)
Anyway Don't make a habit of being late but they are expecting these things every now and then. Unfortunately for you there is no way for you to prove otherwise whether you were on time or not. For all they know you could have been at the front door 2mins before they opened it.
Take it with a pinch of salt. It won't be the first or the last time you'll be late I'm sure.
To stop this happening in the future, if you have a mobile put the store exdir telephone number in it so you can call it if you are stuck outside. Also you'll find that the managers have work mobiles (mine did) aswell as personal mobiles. If they are ok with it get their mobile numbers so you can call the duty manager for the day. They'll send someone down to let you in.