Participatory sensing conducted through mobile phones allows communities to collect useful information from phone users. In this paper, we focus on designing an incentive mechanism to encourage mobile phone users to report traffic status continuously in order to monitor the traffic network without installing traditional sensors. It benefits the power utilities in estimating power demand and managing power supply for the entire network. Under a budget constraint, we address two problems: 1) whom to pay? and 2) how much to pay for each message? We argue the benefit of collecting messages regarding the same event occurred at the same location decreases as the number of collected messages increases. Thus, we propose an incentive mechanism which dynamically adjusts rewards that are paid to users. The incentive mechanism is evaluated on a sample network and the performance comparison result with a RADP-VPC incentive mechanism shows that our proposed mechanism outperforms it with metrics of the total benefit and the total number of mistakes.