A transportation company operates 200 trucks and would like to use a hidden speed monitor device to record the maximum speed at which a truck is operated during the period that the device is installed. The trucks are driven primarily on interstate highways, and the company wants to estimate the average maximum speed for its fleet with 90% confidence and within 2 miles per hour. Using (and explaining) your own estimate for the population standard deviation, determine the number of trucks on which the company should install the hidden speed recording device.