I have been noticing these yellow bumpy things cropping up on sidewalk ramps in the Boston area and decided to do some research about their origin.

These plastic "truncated dome" inserts are known as detectable warnings which serve as a means to notify the visually impaired about the boundaries of street and sidewalk. The visually impaired rely on environmental cues such as texture underfoot, ambient sounds, and physical objects that can be sensed with a cane. Detectable warning systems, such as truncated domes, allows the independent visually impaired person an added measure of safety. There are other ways to make these boundaries detectable by using other textured materials such as brick, or grooved pavement but these methods were deemed not as effective as the truncated domes due to similar surface textures on roads. The DOJ (Department of Justice) in conjuction with the ADA (Americans with Disabilities Act) therefore deemed the truncated dome to be the standard in detectable warnings because this material provided optimal detection underfoot or with a cane. Why yellow, you might ask? People with low vision can perceive the color yellow due to the color's high chromatic (light reflecting) properties more so than any other hue.