Motorcycles take a majority in road vehicle fleet of Vietnam, but there is no data
on the number in current traffic circulation. This study presents a methodology to calculate
this quantity (Mcir). Until now, in all existing works, the numbers of registered motorcycles
are used instead of Mcir in ignoring the fact that after several years many vehicles have been
excluded out of circulation. The use of correct Mcir could lead to better and more economical
transport solutions. The paper also investigates the violations related to motorcycle driver
license and shows that the level of such violations is serious. Furthermore, the paper shows
methods to calculate the average use life, number of current users, and rate of users per one
motorcycle. Especially, the short use life shows one sign for bad technical quality of existing
vehicles. Last but not least, the paper presents a prediction of motorcycle number until 2020