Objectives The objective is to (a) refine and use methods to measure the point prevalence of smoking and of secondhand smoke exposure in moving vehicles and (b) compare these prevalences (1) between two areas of contrasting socioeconomic status and (2) over time.

Methods The authors developed and tested a single-observer method and observed the point prevalence of smoking in vehicles in Wellington, New Zealand. The two observation sites represented high and low areas of socioeconomic deprivation (based on a small area deprivation index).

Results A total of 149 886 vehicles were observed. The mean point prevalence of smoking in vehicles at both sites combined was 3.2% (95% CI 3.1% to 3.3%). Of those vehicles with smoking, 4.1% had children present. Smoking point prevalence in vehicles was 3.9 times higher in the area of high deprivation than in the area of low deprivation (95% CI 3.6 to 4.2). The same pattern was seen for vehicles with only the driver at 3.6 times (95% CI 3.4 to 4.0), in vehicles with other adults at 4.0 times (95% CI 3.4 to 4.7) and in vehicles with children at 10.9 times (95% CI 6.8 to 21.3), with all results adjusted for vehicle occupancy.

Conclusions Observing smoking in vehicles using a single-observer method provides a feasible and objective indicator of the different smoking behaviours, especially around children, within an area. This study further supports the evidence from this country and internationally that adults and children from high-deprivation areas are much more likely to be exposed to secondhand smoke.