New assembly plants cost General Motors Co., Ford Motor Co. or Fiat Chrysler Automobiles NV about $1 billion — the sort of investment companies look to avoid making as a market peaks. And while factories boost jobs, economic gains from building them are being undercut by automation and pressure to compete with lower-wage countries including Mexico.

“This is the nightmare scenario for auto companies, which are being asked to make huge capital investments right before a slowdown in sales,” said Dan Luria, an analyst who has advised the United Auto Workers union. “It seems like hardly the time to spend billions on new plants.” …

After the U.S. auto market’s 68 percent surge since 2009, sales will be roughly flat through 2020, researcher LMC Automotive said in a report last week. After setting a record with nearly 17.6 million vehicles last year, the industry will keep coming up short of that level through the end of the decade, LMC said.