Those little white or yellow bumps that some highway departments use for lane markers. I learn from a fellow noder that an official name for them is Botts dots.

The good thing about them is that they're very visible, even in rain or a little snow, and they let you do a certain amount of driving by braille, if you're into that sort of thing. The bad thing about them is that, where it matters, snowplows scrape them right off.

Usually the dots are about two inches / 5 cm in diameter and rise perhaps a quarter of an inch or 1 cm above the road surface. In Seattle, WA, however (and perhaps elsewhere) some truly turtle-sized ones are occasionally used, 2-3 times as big in both dimensions. (You do not want to run over this kind!)

There are also variants which have a trapezoidal cross-section, with reflectors built in. A few highly-enlightened highway departments will occasionally mount this kind in shallow grooves, with their tops flush with the road surface, meaning that you don't get the driving-by-braille effect, but also meaning that plows don't scrape them off, either.

(On a divided highway using the trapezoidal kind, the reflectors facing the wrong way are red. If you're ever driving along, not paying attention, and you notice that the lane-marker reflectors are all red, and you say to yourself, "Gee, I wonder why I've never noticed that before?", it's because you've never been driving the wrong way before. Pull off or turn around immediately!)

I was amused one day on a Southern California freeway to see some road turtles actually being installed. There was a highway department worker riding in a highway department go-kart scooting along and sticking them down with dollops of epoxy. Evidently the go-kart is the right tool for the job, and I still get a kick out of contemplating the fact that CALTRANS owns a few of what I would otherwise have assumed were exclusively toys for teenage boys.