We shave about 20 minutes off our 40-mile commute by using the carpool lanes.

[GoodTaste]

Meaning of carpool lane in English

TRANSPORT, SOCIAL RESPONSIBILITY
in the US, part of a road that is for carpools only to use:
We shave about 20 minutes off our 40-mile commute by using the carpool lanes.

Source: Cambridge Dictionary

Does "shave" here mean "reduce"? That is, if the usual time needed for the 40-mile commute is 35 minutes, now we arrived at it in 15 minutes because we used the carpool lanes?
Logically, it sounds that we illegally used the carpool lanes - but if you used "we" rather than "I", isn't it legal to use carpool lanes?

 

[emsr2d2]

Yes. Mind you, anyone covering 40 miles in 15 minutes won't care if they can use the carpool lane because they'll have been arrested for speeding!

 

[SoothingDave]

Yes, carpool or "HOV (High Occupancy Vehicle)" lanes require more than one person to be in the vehicle. Usually 2 people is sufficient, but some are for 3 or more occupants.

 

[Skrej]

Meaning of carpool lane in English

TRANSPORT, SOCIAL RESPONSIBILITY
in the US, part of a road that is for carpools only to use:
We shave about 20 minutes off our 40-mile commute by using the carpool lanes.

Source: Cambridge Dictionary

Does "shave" here mean "reduce"? That is, if the usual time needed for the 40-mile commute is 35 minutes, now we arrived at it in 15 minutes because we used the carpool lanes?
Logically, it sounds that we illegally used the carpool lanes - but if you used "we" rather than "I", isn't it legal to use carpool lanes?

Reducing a 35 minute drive to a 15 minute drive isn't 'shaving' - that's over halving it!

'Shave' implies relatively small changes, to me. I'd use some other cutting metaphor for such a substantial decrease in your example. Perhaps 'trimmed', 'chopped', even 'carved', if you want to stick to cutting metaphors.

For example, a runner might 'shave' a few hundredths of a second off a previous record to establish a new one.