It is illegal to leave a running lawnmower unattended on a public sidewalk.

The Law Explained

This hyper-specific ordinance was likely passed after someone carelessly abandoned their rumbling yard equipment, causing a localized hazard. Pedestrians, especially children, were at risk of walking into spinning blades or getting burned by the hot engine. If you need to grab a glass of lemonade, local law demands you turn the machine off first.