How do I make a sprite move randomly in the map?

I have a sprite, a city map. How to make the sprite move randomly on the street, change direction when it hit a the wall?

(object) on impact with wall> (object) set object angle to> choose(self.angle+random(-90, -160), self.angle+random(90, 160))

. . . . . . . .event . . . . . . . . . . action . . . . . . . . . . . . . . . . . . . . . . . . . . . . formula to use . . . . . . . . . . . . . . . . . .