I would think that, on an incremental pollution basis, an electric car trapped in traffic would have an advantage. If a significant fraction of a city's autos would convert to electric, smog would be reduced.

Really, the environmental benefit (or not) of electric cars should be examined  considering the total life cycle cost of manufacturing, using, and decommissioning of the vehicle. This "cost" should include air, water, and other environmental contributions from mining the ore to dumping in the landfill -- along with items consumed during use and maintenance cycles. Don't forget to include the HUGE pollution footprint of tankers and pipelines.

With respect to the "cost" of energy, it requires a certain amount of energy to move the mass of a car and contents from here to there. Is the "cost" of supplying energy lower when delivered one gas tank at a time, charging batteries using large power plant generated electricity (oil, geo, or solar based), or home solar. Don't forget to include building and maintaining the mining, drilling, refining, manufacturing, and distribution system "cost", efficiencies along the way, and final end of life disposal of everything in this chain.

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We remark about electric cars as having a large battery and lots of pollution associated with the battery. An alternative would be using a fuel cell in the car. The "fuel" associated with the cells is easily available and low impact if it leaks into the environment. In terms of "danger" associated with the fuel, it can't be much worse than that of gasoline. If the car is involved in an accident I would much rather have the hydrogen and oxygen quickly explode, burn, or leak into the environment than be faced with a gasoline explosion or fire, or a lithium fire. Recently, one of my customers lost an electric car in a fire. There was no accident. The battery became upset while driving and the car was consumed.