Assume that as traffic density increases along a given stretch of road, there comes a point when traffic begins to slow down. The following table gives the times taken for a car to travel the stretch of road (in minutes) according to the number of cars entering the road per minute.

Cars entering the road	5	6	7	8	9	10	11
Journey time	10	10	11	13	16	22	30

(a)  Copy out the table and add the following rows: (i) total journey time for all cars; (ii) extra journey time as traffic increases by one more car (marginal social time cost); (iii) additional time cost imposed on other road users for each additional car entering the road (marginal external time cost). (See Table 13.3 in the text.)

(b)  Assume that time is valued at 10p per minute. On a graph, plot the marginal private time cost (journey time) and the marginal social time cost.

(c)  Assume that electronic road pricing is introduced. What charge should be levied when traffic density reaches (i) 6 cars per minute; (ii) 8 cars per minute; (iii) 11 cars per minute?

(d)  What additional information would you need in order to work out the socially efficient traffic density on this particular stretch of road?