[coldbrewed]

The heat from vehicles isn't distributed spatially across rooftops/walls/trees where the heat might be dispersed; instead the heat from vehicles is concentrated and radiated adjacent to sidewalks (impacting pedestrians) and asphalt (which is effective at storing and re-radiating heat). Nor is it dispersed evenly throughout the day; congestion during rush hour will cause a spike of heat during the hottest part of the day with greater numbers of pedestrians experiencing that heat. Idling vehicles are also running air conditioning, and all of those idling/air conditioned vehicles will be creating an ambient atmosphere where their AC systems will have to run harder to create the same level of cooling.

As you note solar heating likely dominates the overall heating of the city but I would fully expect that idling vehicles contributes meaningfully to the pedestrian and driver perceptions of heat.

[KorematsuFredt]

BS. Most cars drive on freeways and interstates most of the time, especially the large trucks etc. traffic in residential areas is generally pretty low and it more only during mornings and evenings.

This is just war on cars.

[TomK32]

Cars are used only 5% of the time, the rest of the day most of them sit around. They sit around in the sun, heat up and only disperse this heat during the night. Without those massive metal blocks in our streets, instead of large trees, the air could stay cooler during the day and night.

[milkytron]

In a lot of US cities, freeways/state highways are where a lot of commercial, retail, and entertainment destinations exist. The first homes usually aren't that far away either, and a lot of apartment complexes are built directly on state highways.