Explained: How US emissions rules encourage larger SUVs and trucks

Drivers around the world want more roomy and powerful cars. Rules made under the administration of President Barack Obama make it easier for U.S. automakers to give customers what they want and to keep expanding the size of their vehicles.

Under the federal regulations, crafted with heavy input from Detroit, cars have to meet tougher emissions and mileage targets than light trucks - a category that includes pickups and many so-called crossover vehicles that look like SUVs but have the mechanical underpinnings of cars.

They set easier emissions standards for vehicles that have larger "footprints," measured as the area between the points where the wheels touch the ground.

As a result, automakers can increase the exterior dimensions of many SUVs and trucks while remaining in compliance with federal emissions standards.

The new Cadillac Escalade, for example, is about 7 inches (18 cm) longer than the previous generation. Customers are willing to pay well over $80,000 to drive one.

An 8 square foot increase in the footprint of a vehicle can allow for 2% to 3% more carbon dioxide emissions, according to industry experts.

That can mean the difference between complying with regulations and paying an emissions fine - although many other factors affect whether an automaker's fleet is in compliance.

The shift toward ever-larger SUVs has had a global effect.

"SUVs were the second-largest contributor to the increase in global CO2 emissions since 2010 after the power sector," the International Energy Agency warned in a report in October.

That has effectively destroyed emissions targets set by the Obama administration.

"We didn't expect that SUVs would be so prevalent," said Margo Oge, former head of the U.S. Environmental Protection Agency's office of transportation air quality.