Ratios of traffic modes against cycling varied significantly except walking. During the sampling process, percentages of ratios higher than 1 were 61%, 81%, 44% and 23% for bus, subway, taxi and walking respectively. That means walking was exposed to a lower BC environment on these routes in summer. The walking to cycling ratios of BC exposure concentrations were between 1.42 and 0.63 with an average of 0.88. Meanwhile, walking to cycling ratios were relatively stabilized below the ratio 1. For walking ratios higher than 1, that may be caused by the fact that the commuters were occasionally exposed to particular emissions such as barbecue stalls, tobacco smoke, coal burning and garages when they A 887826 were testing engines, etc. On R1, ratios of traffic modes were relatively low and stable compared to that of R2 and R3. As for larger flow rates of traffic and visitor combined with high tall building density, R2 showed higher variations in traffic modes to cycling ratios. Compared with R1 and R2, walking to cycling ratio on R3 was higher even though no barbeque and other particular emissions were spotted along the pavements. This is due to the fact that R3 contains 3–5 story residential buildings and dense shade tree cover along the two sides of photosystems two-lane road. Vehicle traffic and its trail gas were largely restricted to this narrow route and it\'s difficult for gas diffusion. This was further proved by the between-route comparison. The comparison between every two routes showed that the ratio of average BC of cycling (and walking) along R1 to that along R3 was 1.26 (1.05), and R2–R3 was 1.20 (1.00). It indicates that average BC along busy routes R1 and R2 was slightly higher than that along R3. The dispersion status of the road microenvironment may be a key factor for this situation. But average BC of all traffic modes (except subway) on R3 was the lowest among these three routes because of its lowest traffic density for bus and truck which contributed a large portion of BC to road traffic.