Abstract Cities worldwide struggle to reduce car dependency despite its costs: congestion, pollution, inefficient land use and traffic fatalities. Traffic light timing, often favouring cars, plays a surprisingly powerful yet underexplored role in commuters' choices. We introduce a traffic model simulating how signal timing affects the balance between cars and sustainable alternatives (walking, cycling, public transport). Commuters adaptively switch to their fastest option until equilibrium, where no one can reduce travel time by changing modes. We uncover a traffic light paradox: allocating more green time to cars can increase driving times. Favourable signals attract more drivers, whose collective congestion outweighs individual savings: a manifestation of the price of anarchy in urban mobility. Calibrated to realistic urban conditions, grid-based simulations show that modest reductions in car green time can shift modal share towards sustainable transport, even under network irregularities and non-linear congestion, without costly infrastructure changes. These findings challenge car-centric signal design. Adjusting traffic light cycles lets cities promote active mobility and public transit, supporting sustainable urban mobility goals, including the 15 min city. Our model provides planners with a tool to evaluate signal timing policies, showing traffic lights are not merely flow regulators but powerful instruments for shaping travel behaviour and achieving sustainability.
Cerioli et al. (Wed,) studied this question.