Emissions from electricity generation have seen an overall downward trend in recent years. But emissions from transportation stubbornly continue to balloon, as Americans keep favoring big gas-guzzling cars.

The power sector has gotten some help. The Regional Greenhouse Gas Initiative, a cap-and-trade system covering nine Northeastern states, has provided a market-based incentive for power plants to reduce their pollution.

Now, a coalition of Northeastern states is looking to craft a regional cap-and-trade system for cars.

That’s never been done before. And it raises a host of thorny policy questions.

For one thing, power plants are stationary sources of pollution. But cars are mobile sources that can cross state lines, making tailpipe emissions a trickier problem.

Read the full report from The Energy News Network.