The Catalytic converter was introduced in 1975 to comply with the tightening Environmental Protection Agency (EPA) regulations on auto exhaust. The job of the catalytic converter is to convert harmful pollutants into less harmful emissions before they leave the car’s exhaust system. Catalytic converters typically consist of a ceramic or metal honeycombed monolith substrate that carries precious metal catalysts. The coated substrate is wrapped in an intumescent mat that expands when heated, securing and insulating the substrate which is packaged in a stainless steel shell and fitted into the engine exhaust system. As exhaust gases pass over the catalysts, they promote chemical reactions that convert pollutants into harmless gases and water.
"Hybrid gas-electric cars such as the Toyota Prius and Honda Insight are nearly twice as fuel efficient as other cars, but they produce less than 10 percent as much pollution. Much of the pollution reduction comes from a simple device designed to ensure that the catalytic converter runs at its optimal temperature at all times. If incentives created a market for pollution-reducing equipment, it would be easy for someone to design and sell a catalytic converter heater for existing cars." 
It only works when it's hot enough (400 degrees), which requires your car to go 50 to 60 mph. Traffic congestion lets the catalytic converter cool down, rendering it ineffective.