Fig. 3From: Impact of passenger engagement through road safety bus stickers in public service vehicles on road traffic crashes in Zambia: a randomized controlled trialIllustration of study population and stratified two-arm design. The stratification is done by bus type i.e. big buses and minibuses. The big bus two-arm trial differs from the minibus two-arm trial because the RTSA toll-free bus sticker was non-systematically distributed in the big buses while none of the minibuses were exposed to the toll-free stickerBack to article page