The original plan called for returning the streets and breaking up the super block. That's why 7 WTC was built so small and tall (compared to the original 7) and unblocked Greenwich St. The plan envisioned was to reconnect the WTC complex with NYC. Having traffic flowing through would return the area still (incorrectly) referred to as 'Ground Zero' to normalcy.
So with that plan in hand, they went to work designing and constructing the center. It wasn't until a few years ago, that the NYPD decided that they needed to treat the area like a federal prison and determined that they would need to erect guard booths at every entrance, and restrict traffic flow (oh and guarantee a dozen or so jobs in perpetuity...). This was after the redesigned 1 WTC at the NYPD's request, to be the concrete fortress it is today.
Personally, I think the NYPD's plan to seal off the site is a horrible idea. It closes off the center from the rest of the city, and it gives anyone who visits this heightened security police state mentality. It sends the message that we're afraid and on guard, not the message that we're resilient and tough. Paving roads no cars can drive down is stupid. If the NYPD really wants to lock the site down, they might as well ditch the roads and turn it back into one big pedestrian plaza (hey, at least it would let us put the Sphere back on the exact spot (albeit 1 story lower) where it originally stood!)
The only other example of what the NYPD is trying to accomplish is around the NYSE. A short section of Wall St, Exchange Pl, and a few tiny alley roads were blocked by barriers and turned pedestrian only. Unlike the WTC plan, this works for 3 reasons:
1. The roads were extremely narrow and not conducive to traffic flow. Greenwich St on the other hand is a major thoroughfare or at least it was supposed to be.
2. Wall St is a major tourist destination, but there is no plaza or open public space for these tourists to congregate. By closing the roads, it has created a large public plaza much like closing Broadway in Times Square. They have also replaced the paved asphalt with paving stones to enhance the aesthetic. The WTC on the other hand has a multi-acre open memorial plaza available to the public.
3. The historic NYSE trading floor is only approx. 20 ft off Exchange Pl behind a thin masonry wall. A truck bomb detonated in front of the building would inflict serious structural damage. 1 WTC on the other hand has a 16 story bunker base with a concrete reinforced outer wall designed to deflect blasts. It is also built further off the road to minimize risk. Even the lower windows facing the open roads are reinforced against shockwaves. A truck bomb would do little more than break the glass facade.
Finally, the notion that 1 WTC is a terrorist target is a silly argument to push. While I'm not disagreeing that there are likely people out there who would love to attack these buildings, the same argument could be made for dozens of other unprotected sites around NYC. Are we going to close 34th Street because of it's close proximity to the Empire State Building? Surely we should close 7th Ave to protect the populated Times Square from bombers right? The Brooklyn Bridge is also a NY treasure. We should close it and make it pedestrian only (after passing through a security checkpoint) because god forbid someone could detonate a truck bomb mid-span and destroy that 131 year old treasure. Hell, terrorists could strike anywhere, we might as well ban all personal transportation and force everyone to take public transportation, after having their bags inspected of course.
TL;DR? The NYPD security at the future WTC is "security theater" and is a waste of time / effort. It undoes the progress and vision of the new WTC complex by promoting fear and an increased police state.