Almost half of the cars clogging Park Slope’s main commercial arteries are driving in circles in search of parking, a new traffic study from a transportation advocacy group shows.

While vehicles competing for parking spaces account for only 28% of street traffic on some of Manhattan’s most congested streets, 45% of drivers on the road in this primarily residential Brooklyn neighborhood are searching for curbside parking, according to the study, which Transportation Alternatives will release today.

A lack of parking options translates into lost business, as potential customers grow frustrated circling the block and eventually take their business to other neighborhoods, the study shows. About 15% of parked cars are also illegally stationed in front of fire hydrants, no-standing zones, and ambulance lanes near hospitals.

“People come to the store and double park in the bus lane,” the owner of Slope Sports on Seventh Avenue, Kirsten Marino, said. “It’s a concern for them getting a ticket, and I feel like it reflects badly on me if they remember getting a ticket while they were at my store.”

At a press conference today in Park Slope, Transportation Alternatives will unveil its recommendations for how to relieve what it calls a “disastrous situation” for residents and local businesses.

The study recommends that meters charge more for curbside parking and that residential parking permits be issued.

“We had folks tell us they were circling for up to an hour and a half and then went home because they couldn’t find parking spaces,” the author of the study, Wiley Norvell, said. “There’s tremendous frustration on the part of the drivers.”

A spokeswoman for the city’s Department of Transportation, Kay Sarlin, said the department would not comment on the study or its recommendations.