We test two robotic mowers and find one Not Acceptable

Robotic mowers promise to save time and effort as they rove within a perimeter wire that sets the mowing boundaries. But like many robots in countless sci-fi movies, one of these robotic machines poses a serious risk to those it would serve.

We tested Friendly Robotics' $2,000 RoboMower RL1000 and LawnBott's $2,500 LB3200 Evolution. Both crisscross randomly within their boundaries, reversing direction when they reach the wire or an obstacle and returning to their charging stations when needed. And both use metal blades. When we lifted the 78-pound RoboMower while mowing, its blades stopped roughly 1 second after its wheels left the ground. But the blade on the 25-pound LawnBott robotic mower kept spinning until we lifted it beyond roughly 45 degrees. Even after that, its blade took nearly 4 seconds to stop. In our judgment, either situation could harm an adult or a curious child.

We contacted the U.S. Consumer Product Safety Commission (CPSC) and asked it to investigate the LawnBott LB3200. We also contacted this Italian-made robotic mower's U.S. distributor, which told us that a downloadable software update stops the blade within 1 second after this mower is lifted beyond about 35 degrees. But even if the blade stops more quickly, according to the distributor, it won't stop until you lift the mower beyond about 35 degrees. The distributor says that feature allows the LawnBott to work on steeper slopes. We believe that even with this update, the LawnBott LB3200 Evolution poses a serious safety risk and have judged it Not Acceptable.

Since 1983, all powered walk-behind mowers must protect hands and feet by stopping the blades quickly after the operator releases a handlebar deadman control, as part of a mandatory CPSC standard Consumers Union helped develop. Ride-on machines stop their blades when the operator leaves the seat as part of a voluntary industry standard. Yet no such standard exists for robotic mowers. We believe that all mowers should be required to meet effective safety standards.

As for the RoboMower, its cut was unimpressive, and it sometimes got stuck and didn't return to its charger. We also question the time-saving claims. With both robotic mowers, you're warned to keep children and pets away. RoboMower warns you to supervise its mowing—advice we support for all robotic mowers. For a half-acre lawn, that could mean 4 hours of supervising several times a week, compared with about an hour a week of walking or sitting with a mower.