Slip base (frangible) poles are designed to shear at the base when impacted by errant vehicles. The height of the shearing plane should be 75mm above ground level with a ±25mm tolerance. Too high and the pole may snag a vehicle. Too low and the pole may not slip/frange as designed.
Bolts should be checked and re-tightened regularly. How tight?: 90 Nm for the full nut, 40 Nm for the half nut. (see TCN006-2016).