Is it justifiable for my management to refuse paying me for overtime?"?

As you know I am a transport driver, and in the winter here in B.C the roads can get pretty bad at times.
So from time to time the run can take extra time, meaning extra hours anywhere from 3 to 10 hours on a pay period.
Management no longer wants to pay for those hours.
As drivers we can’t help the extra hours due to road conditions, or frozen equipment.
Is management within their right not to pay those extra hours?
Should we refuse to go when we know it will take extra time to do the run?
I should add our runs are based on a trip rate, but that rate was figured out on good running conditions, winter conditions were never figured in the rate time.