The machine looks like it would be quite stable on level ground. Since the employee was mowing a "drainage easement" (ditch) it appears he either was not following proper usage for the machine or he wasn't properly trained to operate it...probably a little of both. Properly training would include how much of a sidehill the machine could be safely operated on. This was totally preventable with a little forethought and due diligence.
The city of Lincoln has removed from service two three-wheeled mowers, including the one that tipped over which have an old design and are rarely used. The city also suspended mowing around all ditches.
Kohles had been a city employee for about 2 1/2 years, so not really all that experienced on this particular mower, apparently.
One news story says "About a year ago, Opfer (Scott Opfer, manager of Lincoln's street and traffic operations) said, workers told him they were concerned with the ditch Kohles was mowing Monday. Workers were using a different style mower at that time."
So apparently the safety factor of mowing this particular ditch was an issue with the people who actually mowed it before and they made management aware of it. This should have triggered an immediate investigation into what mowers were safe to use...and what ones weren't. Those two particular mowers should have been clearly marked "for use on level ground only" and all the operators should have been made aware of that fact, but I believe it makes more sense to have all mowers capable of safely cutting everywhere. That particular ditch slope should have been reworked too (if possible) since even with the other mowers it seems to have been overly steep for comfort.
Just conjecture on my part here...I believe there's a good chance the guy thought that since the ditch was mowed before, it was safe for that machine...after all, they sent him out there with that machine to mow that ditch. He probably looked at the outlying rear wheels and figured it would be stable, not realizing those wheels would simply move upwards when it started to go over. A fatal bit of judgment on his part, if that was the case.
Someone dropped the ball and a man died because of it.