Abstract: An improved carriage incorporates an automatic braking system that detects and reacts to the presence of a ledge and substantially inhibits movement of the carriage over the ledge. The carriage includes three rolling units pivotally coupled to a base in spaced relation about the periphery of the base. Each rolling unit includes a pair of wheels. A high friction pad is mounted on the base and presented toward the underlying surface for frictional engagement therewith. Movement of any one of the wheels over a boundary edge of the underlying surface causes the rolling unit supporting such wheel to pivot from a normally horizontal rolling position to a pivoted position. Such pivoting movement acts to collapse the carriage thereby lowering the high friction pad into frictional engagement with the underlying surface. A warning system is desirably provided for indicating engagement of the high friction pad and the underlying surface following collapse of the carriage to alert nearby persons of such collapse.