To me, this is another example of bad writing. Your idea makes sense, but the writers went with their option because it provides a lot of big-bang! moments for the show instead. All they had to do was build a small fence where the trucks were, but instead they built a MUCH larger fence at the junction where they needed hundreds (thousands?) of zombies to all make a sharp-right!

And yes, once the small fence was constructed, Alexandria could have made bows and arrows and the whole town would have had a ton of archery practise eliminating every last zombie until everyone had the survival skills that....Rick said he wanted all of them to have.... OOPS...opportunity blown!