Respondentswere providedwith the option “I don't know” as an answer
to the questions aboutWTP amounts to avoid themgiving an irrelevant
answer or for the sake of going to the next questions. If
respondents answered €0 on the WTP general questions, follow-up
questionswere asked about theirmotive for the zero response. Options
for these follow-up questionswere: i) costs should be included in transportation
prices; ii) government should pay all costs to reduce air pollution;
iii) effects of air pollution from road traffic are negligible;
(iv) principally against putting amount of money on health; and
v) other reasons. Options i), ii), and iv) of these follow-up questions,
combined with the WTP of €0, were used to identify a protest vote
(PV), an answer indicating that respondents did not accept the concept
of WTP.