Last spring, an 11-mile stretch of Interstate 394 in Minneapolis previously reserved for carpools became a toll road for solo drivers.

But San Diego officials recently converted many of the lanes to allow solo drivers to use them for a $50 monthly fee.

(Another bill approved this week by the Legislature would open several underused car pool lanes to solo drivers who pay a toll).

And they were not just looking for solo drivers sneaking into the H.O.V. lane.

Mr. DeCroce has introduced legislation to study the effectiveness of the lanes and to relax restrictions against solo drivers.

For a solo driver, on-time arrival comes with a price: nearly $11 per round trip, a toll collected through electronic signals.

Slugging lines are common in some places, where solo drivers pick up a passenger to share the ride and allow use of the HOV lane.

The solo drivers are allowed to use the HOV lanes upon payment of a fee that varies based on demand.

This is difficult for a solo driver with a baby who needs attention, but until air-bag modifications come about, it is vital.

However, with a four-in-hand, the solo driver could handle all four horses by holding all the reins in one hand, thus the name.