A transport study is being undertaken incorporating four cities A, B, C and D. The travel costs between these cities in generalised time units are given below; note that intra-urban movements are excluded from this study: Origin ABCD Origin ABCD A D 1.23 1.85 2.67 A Destination 8 6 B 1.23 2.48 1.21 Roadside interviews have been undertaken at several sites and the number of drivers interviewed is shown below together with their respective origins and destinations. Blank entries indicate unob- served cells. B C 6 1.85 2.48 Destination 18 1.44 C 1 - D 6 2.67 1.21 1.44 - D 248 8 Assume now that a gravity model of the type Tij = RS;F is to be used for this study area with only two cost bins. The first cost bin will cover trips costing between 0 and 1.9 and the second trips costing more than 1.9. Calibrate such a model using a tri-proportional method on this partial matrix. Provide estimates of the parameters R, Sj and F, and of the missing entries in the matrix, excluding intra-urban trips. Are these estimates unique?