One shortcoming of most computer mapping systems (Google, Rand, Mapquest) to calculate distance to a box, as Atlas points out, is the one way street consideration. I know of one firehouse (I am sure there are more) located on a one way street in the Bronx whose companies go against that traffic for about 200 feet to reach a major intersection when responding to structural fires in that direction.
The computer mapping systems will add as much as a quarter mile to their actual trip, routing them around several blocks to get back to that intersection to keep them always flowing with the street traffic direction. The companies would be long operating at the box by that time. I find it useful sometimes to do a reverse map function (from the box to the firehouse) to get a truer calculation.
The pre computer solution to box assignments was the "wheel on the map" at the Alarm Assignment Division at headquarters. A device (similar to a wheeled, rolling eraser, if anyone remembers those) calibrated to and simply rolled along the City street map determined the distance from each house to a box. The operators knew the traffic patterns of each firehouse and how they responded. They also considered major traffic intersections and potenial choke points along the possible routes. Where there were equal distances they would actually go out and recon the routes of the companies involved and/or contact chief or company officers for resolution. When streets were closed for temporary reasons (construction, school "play" streets, ect.,) an order was published to accomodate the situation. All officers and personnel involved took this responsibility seriously, as I am sure they do today, to simply ensure that the right companies responded to give the fastest fire protection to the citizens of New York. Professional.
The proof came every day. Units arrived as assigned, all conditions equal. It was fun to watch the results when some of the first alarm companies were out elsewhere. Those special called by telphone by the dispatcher to fill in the box assignment also would arrive as the wheel predicted. Amazing!
With all our technology today, seems we could could, somehow, at least match the wheel in accuracy.