Increasingly-aggressive emissions targets will soon force automakers to build more plug-in electric cars.

Norway has proposed all but eliminating new internal-combustion car sales by 2025, and a consortium of regional governments called the International ZEV Alliance has proposed to do the same in its jurisdictions by 2050.

But even given those aggressive timetables for new car sales, how long will it be before a large part of the world's billion-plus vehicles are using grid electricity rather than hydrocarbon combustion?

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If laws were passed tomorrow to limit the number of new internal-combustion cars, it would likely take almost two decades to bring half the overall fleet in that jurisdiction to electric propulsion.

No law outright banning operation of any vehicle with a tailpipe has been proposed anywhere, as far as we know, even in Norway.

As fleet-management company FleetCarma points out in a recent blog post, it takes a long time--decades--for even 50 percent of cars to comply with a new law.

Chevrolet Bolt EV concept, 2015 Detroit Auto Show

Chevrolet Bolt EV concept, 2015 Detroit Auto Show

Once a new law is passed--one example might be the earliest vehicle emissions standards, passed in 1971 to take effect in 1975--the first step is to start redesigning non-compliant vehicles.

For a law passed at the end of 2015, the earliest manufacturers could put vehicles in showrooms would be 2019 says FleetCarma, based on its own experience with consulting.

But that's just for one model; the process of design and manufacturing and supply-chain preparations must be repeated until every model in a carmaker's lineup is replaced.

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It generally takes manufacturers five to six years to turn over their entire lineups.

That means if the first compliant vehicles launch in 2019--as 2020 models--the overhaul wouldn't be complete until the 2025 or 2026 model year.

And once all new vehicles are in compliance with a given law, it will still take a long time for them to replace older vehicles already on the road.

Nissan IDS concept, 2015 Tokyo Motor Show

Nissan IDS concept, 2015 Tokyo Motor Show

The average age of a car on U.S. roads is over 11 years--a record high.

Assuming that age remains the same going forward, it would take until at least until the 2037 model year--11 years after compliance is achieved for all new cars--for 50 percent of vehicles on the road to comply.

Accounting for sales in model years 2020 through 2026 though, FleetCarma says that 50-percent figure could be achieved by model year 2034.

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That means if a law eliminating new internal-combustion cars was passed today, it would take until 2034 for half of the U.S. fleet to turn over.

That's something to keep in mind as policymakers look to increase the efficiency of new cars.

Because the longer it takes to decide on new policy, the longer it will take for the effects of that policy to be felt.

[hat tip: Matthew Klippenstein]


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