Speed data collected on an urban roadway yielded a standard deviation in speeds of 4.8 m...

Speed data collected on an urban roadway yielded a standard deviation in speeds of 4.8 mi/h.

(a) If an engineer wishes to estimate the average speed on the roadway at a 95% confidence level so that the estimate is within 2 mi/h of the true average, how many spot speeds should be collected?

(b) If the estimate of the average must be within 1 mi/h, what should the sample size be?