The article “Evaluation of Criteria for Setting Speed Limits on Gravel Roads” (S. Dissanayake, Journal of Transportation Engineering, 2011:57–63) counted the numbers of vehicles of various types on several roads in the state of Kansas. In Riley county, 13 out of 67 vehicles on Walsburg Road were heavy vehicles, while 32 out of 91 on North 52nd Street were heavy vehicles. Find a 90% confidence interval for the difference between the percentages of heavy vehicles on the two roads.