Methods are provided for preparing a bitumen product having improved properties for use in asphalt road paving. In a preferred embodiment, polyphosphoric acid is added to heated bitumen and stirred for a period of time. After addition of the polyphosphoric acid, one or more sulfur-crosslinkable elastomers and one or more sulfur donor coupling agents is added to the heated bitumen and mixed. The polyphosphoric acid, sulfur-crosslinkable elastomers and sulfur donor coupling agents may also be added to separate quantities of bitumen, and the bitumens may be combined prior to use to obtain a final bitumen product having the desired characteristics.