Fundamentals of Transportation/Mode Choice/Problem2

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TProblem
Problem:

Prior to the collapse, there were two modes serving the Marcytown-Rivertown corridor: driving alone (d) and carpool (c), which takes advantage of an uncongested carpool lane. The utilities of the modes are as given below.

U_{d} = -t_{d} \,\!

U_{c} =-12 -t_{c} \,\!

where t is the travel time

Assuming a multinomial logit model, and

A) That the congested time by driving alone was 15 minutes and time by carpool was 5 minutes. What was the modeshare prior to the collapse?

B) How would you interpret the constant of -12 in the expression for Uc?

C) After the collapse, because of a shift in travelers from other bridges, the travel time by both modes increased by 12 minutes. What is the post-collapse modeshare?