This isn't really a spoiler so much as a tip for everyone, so I'll go ahead and tell you that when you're blocked out of an area, it often means that you're supposed to use the metro system.The city sections of the game are annoying me a bit -- they did a nice job of making it very mazelike and blocking out out of areas you aren't supposed to be able to reach, but the cost is that you don't have that "I can literally go anywhere" feeling like in Oblivion.
I also find I get lost easily, particularly in the vaults...
Regarding my previous comments, I'll also add that I've found it vitally important the puny 10mm pistol be phased out as your primary sidearm. That 6 damage is pretty much nothing, and certainly not the weapon to use on the big ants. If you can't find a better weapon and you're feeling discouraged, then heed this spoiler:
keep following the main storyline and you'll soon have more guns than you know what to do with