Webinar Question and Answer Transcript

Autonomous Vehicle Assignment and Routing in Congested Transportation Networks
(October 25, 2018)

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Q. Why do you use system optimization and not user equilibrium?

Dr. Zhou: So, traditionally, when we find user equilibrium—which means a person is driving the vehicle and in the future we have the vehicle supply and we have the user request from origin to destination. We have to distinguish the supply side and the user demand side. That is why we did try to optimize that vehicle supply to satisfy the use of demand. In the near future of automated driving, we do not have a clear use of equilibriums.

Q. Why do you select five depots to get these results? Were other questions or other numbers of depots tested? What results did they provide?

Dr. Zhou: That is a very good question. We want to show here in the future why we considered different numbers of vehicles. Here we take a very simplistic approach assuming in that small network there are five depots of automated vehicles. We can also assume that each household owns one automated vehicle. In that case, each household itself becomes a depot. This is for illustration purpose. We can try to change the number of depots and do some system analysis.

Q. What about other vehicles in the road network—trucks and buses? How will they impact the model?

Dr. Zhou: That is an excellent question. Throughout our preliminary results here, we have assumed 100% AV conditions. If we have other vehicles in the zone network such as buses or trucks, the marginal calculations can be quite complicated. Then we have multiple crosses of vehicles and users in the system and in the plane reaching those different crosses for sure will impact the model and we would need to have a more complicated consideration along this line.

Q. In the presented scenario, it seems like there will be increased VMTs. Any observation or comment on that?

Dr. Zhou: This is true. The increased VMT could be due to heading cost, if you know the public transit methodology for the vehicles to serve different ship requests. What we try to show here is a trade-off or balance between many different factors in the future. That heading cost—associated with VMT—that cost of operating the vehicle—the cost of maintaining the vehicle basis and also the use and benefit in terms of waiting time. The calculation here could be much more complicated compared to our traditional one vehicle, one passenger policy in traffic assignments.

Q. In my opinion, the MPR=1 scenario will not be a lasting state. This scenario will appear in some specific conditions or special time periods. If CAVs, MPR get to 100% totally, CAV will be a kind of quasi-public transport. People will just be passengers rather than owners. So I think that the scenario that MPR to be 1 or 100% just occurs in some control area or time period. Do you have any comments on that?

Dr. Zhou: Thank you for your excellent question. MPR—here we have market penetration grid. So yes, we try to show what the idea would be of 100% market penetration of a vehicle condition. In the future, we’ll be providing interest to look at early adoption of automated vehicle and the calculation of the mixed condition with, say, of 30% automated vehicle and 70% human-driven vehicle would be quite interesting. Using today’s presentation, we tried to highlight—we do have a way mathematically to calculate the system of optimal traffic conditions, even automated vehicles over VMT.

Q. Was zero occupancy trips included in the calculation to access cost and congestion? As vehicles are called between pickup and drop-offs, there will be zero person trips.

Dr. Zhou: This question is related to my earlier comment of bad headings. What we try to say is when a vehicle carries zero passengers traveling between one location to another location, we have to assume that one passenger to the next location to serve the next passenger, we do consider this zero occupancy trip cost. In the current stage, those costs are limited some more, but when we try to consider the traffic congestion, a zero occupancy trip should be priced at a certain range to reflect the congestion cost or to reflect the additional cost of AV to our system-wide travel costs.

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