What's up with Broadway?

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If you look at a map of Manhattan, NYC, you will notice that all the streets are planned in a grid. The streets are planned in straight lines. The ones going from the top of the map in the pic are the avenues, and the ones join across are streets. They are all numbered.

In the second pic you'll see all the areas. You will probably also notice that the streets of lower Manhattan ant Greenwich Village are not on the grid. You will also notice the street going diagonally across the whole map, that street is the famous Broadway.

Take a look at this short film to see the explanation as to why this is so!

Pic1 from here.
Pic2 from here.