Expanding road infrastructures is a common remedy to traffic congestion. However, it is recognized that widened roads spur additional travels and divert trips from public transportation services. In this sense, road expansion may increase the travel costs of both auto commuters and public transport users. This phenomenon is referred to as the “Downs-Thomson paradox (D-T paradox)”. Apart from commuter’s modal shifts, public transport operations also play important roles in determining the effectiveness of a road capacity policy. To seek better joint road network investment and public transit operating strategies, this thesis investigates the effect of road capacity policies and responsive transit services and their relation in an integrated bimodal context.

This thesis shows that a ch...[ Read more ]

Expanding road infrastructures is a common remedy to traffic congestion. However, it is recognized that widened roads spur additional travels and divert trips from public transportation services. In this sense, road expansion may increase the travel costs of both auto commuters and public transport users. This phenomenon is referred to as the “Downs-Thomson paradox (D-T paradox)”. Apart from commuter’s modal shifts, public transport operations also play important roles in determining the effectiveness of a road capacity policy. To seek better joint road network investment and public transit operating strategies, this thesis investigates the effect of road capacity policies and responsive transit services and their relation in an integrated bimodal context.

This thesis shows that a change in the transit operator's objective function can circumvent the D-T paradox. The paradox is inevitable in the traditional case of a social welfare-maximizing operator subject to a binding self-financing constraint. On the contrary, a profit-maximizing operator is more inclined to improve the service so that the D-T paradox can be avoided. In a more general setting with elastic total travel demand, transit crowding, and imperfect substitutability between automobile and transit trips, this thesis reveals that in all regimes the full price of transit declines only if the full price of driving falls as well. Road expansion leads to deteriorated transit service in the self-financing regime, but the D-T paradox occurs only when automobile and transit trips are nearly perfect substitutes.

As the major focus of existing two-mode analysis is the transit service with the dedicated right-of-way, this thesis contributes to the literature by modeling and analyzing the cross-modal congestion effect for buses sharing the same road with normal cars. Since traffic velocity decreases with the total number of vehicles in the system, increasing bus fleet size will exacerbate traffic congestion and in turn may dwindle down the service quality. Thus, a bus operator would have to make its decisions on account of both direct and cross-modal congestion externalities.

In view of the high construction cost and space limitation associated with road expansion projects, the efficient use and allocation of the existing road capacity is crucial. The last part of this thesis explores the effect of dividing the roadway space to different travel modes. To understand the interaction between the central planner (for system benefit maximization) and the bus operator (for its own benefit maximization), this thesis examines how the bus operator responds to space changes and how the system benefit changes with the roadway space allocation. With the responsive bus service, the condition under which expanding bus lane capacity is beneficial to the system has been established.