Before addressing the WTP questions, respondents received questions
on their perceived health. This was based on the standardised
and validated Health Survey RAND-36 (Hays et al., 1993) to obtain general
health score. In this part, respondent's concern regarding the specific
pollutant was also addressed. In the second part of the questionnaire,
a brief description of the health effects related to road traffic air pollution
and noise was provided. We only presented main health effects
for which authoritative reviews indicate sufficient scientific evidence
related to the pollutants. The third part of the questionnaire assessed social–
demographic information such as age, gender, education, household
net income per month, and respondent's level of environmental
concerns.