In crowded cities like Seattle, employers are coming up with creative ways to get employees out of single-occupancy vehicles when they commute. For big companies, like Microsoft and Seattle Children’s Hospital, the strategy includes private shuttle buses that ferry employees from pick-up zones to work and back.

Private shuttles can be controversial. Critics have raised concerns that the shuttles insulate working professionals and cause delays for other commuters. Those reservations were the impetus for a new study by University of Washington researchers that explored whether private shuttles using public bus stops delayed regular transit service. The researchers found that the shuttles largely did not affect public transportation when they used public transit stops.

Last year, the Seattle Department of Transportation and King County Metro launched a pilot program allowing the private shuttles operated by Microsoft and Seattle Children’s Hospital to use select public bus stops throughout the city.

Researchers with UW’s Sustainable Transportation Lab and data company Swiftly used trackers on public buses to study service on nine of the bus stops for six weeks before the pilot launched and six weeks after. They compared service to those stops with others that were not included in the pilot program.

They found that bus reliability remained the same across the stops, on average. There was one stop on Sand Point Way where public buses tended to arrive two to three minutes late after the pilot started but it isn’t clear whether the delays are a result of private shuttle use. The study’s authors said further research is needed to understand what is happening at that stop.

Microsoft and Seattle Children’s pay a monthly fee to use the stops, which could provide another transit revenue stream if the program were expanded. The goal of the pilot is to help transportation officials determine whether private employer shuttles can share public transit stops without disrupting service. If the pilot is deemed successful, it may be expanded to include additional employers in the area that operate shuttles.