I would of gone with the web,  it would burn yes, but fire burns wood and varnish of any kind is easy to catch fire. With the conditions of Waterdeep, the carriage certainly would of been varnished and painted, so at least a fair chance of the carriage catching fire and slowing everything down. *shurgs* A nice big cube of spider silk that flash burns means a high temperature, thus other things take fire damage. But, anyway... Don't forget the strength check for pulling the wings, I mean wheels off of the fly, I mean carriage. ;)