First factor was Safety and second factor was Environmental.
Source: www.fueltankaccessories.com
These are not new concepts, as some might think. The safety issue was based on potential fire hazards, while the environmental stemmed from the vapors escaping and possibly affecting the ozone.
So with this in mind there were several additional factors directly related to the safety and environmental concerns. They were trying to reduce fuel leakage, from when a vehicle rolled over.
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Another reason will be, to prevent fires from fuel spilling out and also to prevent fires sparking from careless accidents like smoking around the tank.
So the evolution of the fuel cap had actual science behind it.
Additionally the government increased a variety of regulations. These were aimed at not only reducing pollution but cause by the caps, but reducing engine emissions as a whole. As the needs increased so has the technology to include Onboard Diagnostics (OBD) and other means to control, detect, and limit the release of potentially harmful vapors and such.
