In recent years, many urban areas have increased their investments in public transportation systems, aiming to reduce traffic congestion and pollution. Proponents argue that better public transit options will encourage more people to leave their cars at home, thus leading to a significant drop in vehicle emissions. Opponents, however, point out that unless cities also implement policies to restrict vehicle usage, simply improving public transportation may not have the desired impact.
Each side presents their reasoning: the proponents claim that enhanced public transit will automatically lead to fewer cars on the road, while the opponents assert that without restrictions on vehicle use, the number of cars may not decrease significantly.
Based on this debate, which of the following points most seriously undermines the proponents' argument?