Bringing EPA and CARB standards for light-medium duty diesel engines in-line with the EURO-IV standards will allow a large amount of small capacity diesel engines (and vehicles) currently sold outside the United States to be legally sold to Americans.
Modern diesel engines are extremely popular outside the US and much more environmentally friendly than the loud and thirsty diesels that Americans are used to. Modern diesels are quiet, economical and popular.
Some UK vehicles for example (average MPG, not highway; UK MPG converted to US): A Ford Fiesta 5dr 1.6L Diesel can average 50 US MPG. A Jeep Wrangler 4dr 2.8L Diesel can average 23 US MPG. A Chevy Cruze 5dr 2.0L Diesel can average 30 US MPG. Even a 5000+lb Range Rover 4.4L Diesel can average 25 US MPG.
Think $/mile, not $/gal.