When a car hits a sign post and knocks it to the ground, most of us assume the car was going fast enough to completely uproot the pole from the ground.
But falling down is exactly what many posts are made to do under impact. In fact, they’re designed to break in two.
As explained in a blog post for the podcast 99% Invisible, many roadside posts actually use a piece of hardware called a slip base, which allows the pole to break in a way that helps save drivers and reduce damage to cars during an accident.
Posts that use slip bases are split into two pieces, joined together just above the ground.
The upper post holds the sign or electrical wire, and has a plate at its base. The bottom piece, which is installed in the ground, has a plate on its top. The two plates are joined using bolts — but those bolts are designed to give way at certain threshold of impact.
When a car slams into the post, the movement of the plates cause the bolts to break or dislodge, thereby disconnecting the two pieces, as seen in this retro demo video:
The bottom piece stays in the ground, allowing the car to drive over it, while the top one disconnects. If a sign is held up by two poles, the slip base also allows a car to knock one post out of the way and keep driving, rather than getting lodged underneath or stuck between the two. Either way, the collision isn’t as abrupt, and the impact on the car and its passengers is less fatal.
Though this system might sound dangerous — since it seems like the post could simply fall onto the car — the slip base system is designed with torque that causes the top piece of a post to get knocked out of the way, rather than falling right onto the car.
Some slip bases are even built with inclines to accommodate the anticipated direction of impact (on a one-way street or highway, for example).
So the next time you walk by a sign post on the footpath, look down — you’ll likely see some kind of slip base system. The main clue is a set of bolts that are clearly visible above the ground, silently protecting your life.