A road traffic sign in the US has seemingly been hacked by mischievous cyber-pranksters to display a message that reads: 'Trump has herpes'.

Drivers along the busy I-80 road in Davis, Northern California, would have seen the surprising message displayed on the large digital board with the derogatory claim about the controversial president late on Thursday 3 June. The first reports about the hacked sign began appearing on Twitter just before 11pm local time.

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Authorities do not know how it was hijacked, as the signs are "locked with a secret password."

A spokesperson for California's transport authority (Caltrans) said the sign was placed at the location to warn drivers about traffic delays for a bridge-construction project, according to CBS Los Angeles.

While most drivers who would have seen the eye-opening message were likely to either do a double-take in disbelief or perhaps drive off with a chuckle, Caltrans are not seeing the funny side and are investigating the matter, as tampering with equipment is a felony. The traffic sign has since been fixed.

Traffic road signs appear to be a popular target for hackers looking to cause mayhem and this is not the first case of a sign being exploited to attack Donald Trump.

In May 2016, over America's Memorial Day weekend, a number of road signs along the I-30 in Dallas were hacked to display politically-motivated messages including: "Donald Trump is a shape-shifting lizard" and another that said "Bernie for President."