perhaps, at bus stops, people should hold up signs indicating where they would like to go. lone drivers who wish to use the car pool lane could then select riders and qualify for the car pool lane . drivers save time by using the car pool lane, riders save bus fare. everyone benefits.
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all you need are two people in the car to use the car pool lane. 

https://hidot.hawaii.gov/highways/files/2015/04/HOVContraFlowFactRideshare042815.pdf