We were standing on an escalator at the mall last weekend and noticed, for the hundredth time, those little bristly brushes running along the sides at foot level. We finally looked it up. It turns out escalators are one of those everyday objects that are almost entirely designed around things that have gone wrong.
Those short black bristles you see brushing against your shoe are called skirt deflector brushes, and they've been standard since roughly the 1990s. They don't clean anything. Their job is to keep your foot, your shoelace, or the hem of a long coat away from the tiny gap betwene the moving step and the stationary side panel. That gap is where things get pulled in and pinched, and historically it caused a lot of injuries, especially to kids in soft rubber shoes. The brushes give you a physical nudge before you drift too close. Once we knew that, we couldn't unsee them. Every escalator, everywhere, has them now because a lot of very unpleasant lawsuits made them mandatory.
The bright yellow borders painted around each step aren't just for visibility. They're marking the demarcation zone — the exact edge of the step where it meets the next one. When two steps are flat at the top or bottom of the escalator, the yellow lines from adjacent steps line up next to each other and look like one continuous strip. As soon as the steps start to separate into the staircase shape, the yellow lines pull apart visually, and your eye reads it as "okay, this is stairs now, adjust your feet." It's a tiny piece of visual UX that your brain processes without you noticing. The color yellow specifically was chosen because it stays high-contrast under fluorescent lighting, incandescent lighting, and daylight coming through skylights — engineers actually tested this.
At the top and bottom of every escalator, there's a metal piece with teeth that looks a little like a comb. It's literally called the combplate. Each step has grooves in it, and the combplate's teeth slot into those grooves as the step flattens out and slides underneath. If anything gets caught — a pebble, a coin, a shoelace — the combplate is designed to trigger a safety switch and stop the whole escalator. Next time you're on one, look down as you step off. You'll see the teeth of the combplate meshing perfectly with the grooves of the step. It's a small mechanical ballet that's been running since roughly 1921.
If you've got a favorite piece of everyday engineering hiding in plain sight, send it our way — we're always collecting these for future Fridays.
— Alexander & Amanda @ SBATC
