READ THE ARTICLE BELOW..
http://www.leadzleadz.addr.com/article.html
and then
"Please read this aticle carefully. The author believes that variable tolls, et, different tolls during peak and off-peak hours is a sensible way to deal with the problem of traffic congestion in urban areas. Use supply-and-demand analysis, including an appropriate graph or graphs, to clarify how such a policy would work".