Some personal injury law firms now automatically target online ads at anyone who enters a nearby hospital's emergency room and has a cellphone. The ads may show up on multiple devices for more than a month. sshepard/Getty Images hide caption
geofencing geofencing
Stories About
geofencing
Companies are trying geofencing, which uses GPS and radio frequency identification to set up a virtual, wireless perimeter so that cellphone users in that area receive messages or advertisements on their phones. Rawpixel/Getty Images/iStockphoto hide caption