I see trailers w/ painted lines on the tires. What's the purpose?

Easier to see if tires are turning when on the tundra in the winter…..

 

Often wondered this myself.

 

It’s extremely handy in winter up here especially running Super-B or multi wheel setups.

 

What Cat said, those brakes freeze solid in the winter and one of the axles might not be turning. A light load parked on an iced over parking lot will make it hard to feel the locked wheel. The paint offers a nice quick visual check before you get out on the road and either ruin a good tire or have to get under your trailer on a busy roadway shoulder and beat the offending shoes into submission.

 

But I see them a lot down here in the south. Mainly on rattle can chassis’s.
Just saw the above comment about brakes locking up from sitting or temps. Makes sense now.

 

Especially in the dark.

 

One winter, I was snowed-in at a truckstop in Rapid City for 2 days. There was an old guy there who had been driving the northern plains for nearly 50 years, and he told me he marked his trailer tires so that he could see if they were turning freely after being parked when it was about -25°F outside; he pulled forward, checked the left side, then the right side in his mirrors.
He also mentioned that most of the old truckstops around there had old telephone poles laying flat around the borders of the lot for a reason; the lot was graded so that it was just slightly downhill to the pole. That way, you could back your trailer tires up to the pole, and leave your brakes un-set while you went in to eat (with a chock, I assume...).

 

Still the same principle, those chassis are light as hell and wouldn't put up much resistance if only 1 set of wheels isn't turning.