To the Editor: Joan Claybrook makes it clear in ''Crash Test Dummies'' (Op-Ed, Jan. 28) that there have been many mistakes made regarding safety testing of infant car seats.While avoiding mistakes in the future is important, there is an even...o the Editor: John Tierney suggests that the 1978 fuel-economy standards made vehicles lighter and thus less safe. But the fact is that after the standards took effect, 85 percent of fuel economy gains came from gas-saving technologies, not...