The marginal congestion cost in transportation systems refers to the extra cost added to the overall system when an additional traveler joins, leading to longer queues. Unlike some costs like travel time, you can't pinpoint these costs on individual roads. If there's a queue on a specific road segment, it doesn't mean everyone using that road will induce extra delays to others. Only those going through the bottleneck, using up more capacity, contribute to the added delay.
Additionally, the total extra delay depends on how many people the extra traveler surpasses in the queue. This amount depends on where the new traveler joins the queue and the spill-over effects at different merging points. In this study, we look into these factors and their impact on traffic management systems. Surprisingly, we show that, in theory, the total congestion cost can decrease with more demand.
The marginal congestion cost is relatively high compared to other marginal costs as safety. However, the extra cost for a vehicle is quite low. The overall significant cost comes from small delays experienced by many travelers.
We think that it is crucial for various stakeholders, like modelers and governments, to understand what causes these marginal costs in transportation systems. Only then adequate solutions can be proposed. Importantly, these costs don't spread evenly across the network but concentrate on a few roads with very high costs. Creating awareness of these patterns is essential for making informed decisions and managing transportation systems effectively.