The Batman Challenge

It’s a cold, dark (k)night in Gotham City and Batman’s sign has been illuminating the sky. Something bad is happening and almost certainly Joker is behind this.

It’s Halloween and lots of party-goers have dressed up as Joker (Joaquin Phoenix has done his magic again).

How can Batman know who is the real Joker?
Taunting Batman with his devious mastermind Joker has left him this message.

I left a big vat of poison somewhere. If you don't find it in the next 24 hours, the whole of Gotham City will be infected.
P.S.: You will have to use optimization for this one I am afraid.

The Joker
The Joker has left Batman with a map of Gotham on which a Bat-Signal has been drawn. The poison is the point on the Bat-Signal on the map which is closest to Batman’s position right now.
Which point is this?

Help Batman find out!

First person to email us with a result file will get a prize that will be announced on our Twitter account on November 1st at 00:01.