Sucks that you got caught. Security gards suck!
Which highway are you talking about? If it's the one right next to the river, yes you can but there are gates at the entrance. On my first visit we got in the grounds by there. On my second and third visits, I got in by the one on the front fence becaue the gate was closed. What a nice walk that was!
You just have to turn right (or left, depending which way you come from) and you will see the main entrance to the factory site.
Good luck! The biggest building is the coolest! It's really worth the trip.