Traffic delays. The Texas Transportation Institute (tti.tamu

.edu) studies traffic delays. They estimate that in the year 2000, the 75 largest metropolitan areas experienced 3.6 billion vehicle hours of delay, resulting in 5.7 billion gallons of wasted fuel and $67.5 billion in lost productivity. That’s about 0.7% of the nation’s GDP that year. Data the institute published for the year 2001 include information on the Total Delay per Person (hours per year spent delayed by traffic), the Average Arterial Road Speed (mph), the Average Highway Road Speed (mph), and the Size of the city (small, medium, large, very large). The regression model based on these variables looks like this. The variables Small, Large, and Very Large are indicators constructed to be 1 for cities of the named size and 0 otherwise.

a) Why is there no coefficient for Medium?

b) Explain how the coefficients of Small, Large, and Very Large account for the size of the city in this model.

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Dependent variable is: Delay/person R-squared s = 6.474 with 686 = 62 degrees of freedom 79.1% R-squared (adjusted) = 77.4% Source Regression Residual Sum of Squares df Mean Square 9808.23 5 2598.64 1961.65 41.9135 F-ratio 46.8 Variable Coeff 62 SE (Coeff) t-ratio P-value Intercept 139.104 16.69 8.33 0.0001 HiWay MPH -1.07347 0.2474 -4.34 0.0001 Arterial MPH -2.04836 0.6672 -3.07 0.0032 Sma11 -3.58970 2.953 -1.22 0.2287 Large 5.00967 2.104 2.38 0.0203 Very Large 3.41058 3.230 1.06 0.2951