The purpose of the distance and durability vehicle requirements is to assess the aging or deterioration characteristics of the mechanical systems which influence motorcycle emissions. The rationale for proposing the 12 hour soak requirement following each 12 hour increment of motorcycle distance accumulation is presented. This procedure for motorcycles deviates from the light duty procedure which allows continuous operation. It is considered that the motorcycle procedure provides a more realistic simulation of the motorcycle deterioration process without additional cost or time over the LDV procedure due to the substantially shorter distance accumulation required of motorcycles.