Diesel-driven road users in other US states on Wednesday hove their sighs of relief as California backtracked its diesel emission rules by delaying implementation.

California, the only US state having the right to adopt its own emission regulations, passed in December last year the toughest standards in the country and all the other states are allowed to follow suit or opt for federal standards.

The relieved ranged in many states from drivers of trucks and buses to operators of heavy-duty apparatuses like standby generators and construction rigs.

The Associated General Contractors of America estimated that as many as 32 states were closely following the footsteps of California in enforcing the first-time emission limits for in-use fleets of diesel-driven equipment.

But in an unexpected policy turn allegedly prompted by the ongoing recession, the California Air Resources Board (ARB) has ordered modifications to the emission rule specifics worked out in April.

If implemented without delay, around 1 million trucks and buses countrywide will have to be retrofitted or replaced altogether while some older models will have to be phased out in due time.

The initial ARB standards require retrofits with smog filters or cleaner engine technology beginning 2011 and with soot filters for all trucks and buses beginning 2014.

These rules would also apply to out-of-state motor vehicles driven in California.

All tallied, the new rules would cost the trucking industry alone US$5.5 billion.