Great question—it’s one of those details you don’t notice until a thin heel suddenly dips between two slats. In most modern street benches, the typical gap between wooden or metal slats is around 3/8 inch to 5/8 inch, roughly 10–16 mm. That is usually small enough to keep a standard stiletto tip from sinking too far, but not always.
A lot of older wooden park benches or low-cost public seating have wider spacing, sometimes 3/4 inch to 1 inch, about 19–25 mm, and wood can shrink over time, making the gaps even less predictable. Stiletto heel tips are often only 8–10 mm across, so they can slip into a larger gap, especially if you step at a slight angle or put your weight on one spot. When the tip drops through and the wider part of the heel catches the edges, it can definitely feel stuck. It’s not usually permanent—you can normally lift straight up or tilt the heel—but it can scratch leather, chip the heel tip, or leave you wobbling.
Urban bench design has moved toward smaller gaps, solid metal panels, or narrow perforations partly for comfort and partly for safety. Even so, I’d avoid stepping directly on slats in thin stilettos. If you have to step across a bench or sit on one, place your weight on the frame or solid supports, or step on the center of a slat with your whole foot rather than the heel tip. So yes, high heels can get stuck, but with a little awareness, it’s easy to avoid turning a bench into a heel trap.