People who are well aware of the health effects of road-traffic air pollution and noise have a higher WTP.
People who report to be very concerned about the environment in general, and/or very concerned about the effects of air pollution and/or noise specifically, have higher WTP.
People who report to be very annoyed by road-traffic air pollution or noise provide higher WTP.
People who report to be highly sensitive towards the effects of air pollution and/or noise, and/or have difficulty relaxing in polluted or noisy places, provide higher WTP.