For a road sign to be a road sign, it needs to be placed in proximity to traffic. Inevitably, it is only a matter of time before someone drives into the pole. If the pole is sturdy, the results may be fatal.

The 99% Invisible City, a delightful new book about the under-appreciated wonders of good design, explains a solution. The poles that support street furniture are often mounted on a “slip base”, which joins an upper pole to a mostly buried lower pole using easily breakable bolts.

A car does not wrap itself around a slip-based pole; instead, the base gives way quickly. Some slip bases are even set at an angle, launching the upper pole into the air over the vehicle. The sign is easily repaired, since the base itself is undamaged. Isn’t that clever?