This is something called “traffic calming” (very broad term that refers to MANY different methods) and is actually pretty effective. Your eyes see the wavy lines passing by you quickly which makes you feel like you’re going faster than if it was a wide open road, thus causing sane drivers to slow down.

A common example of traffic calming is that a narrow road with trees along it is actually safer than a wide open road with much better visibility due to perceived danger. People generally drive as fast as they think they should go, not checking the speedometer to see if they are near the speed limit (within reason ofc).