This is a sign, originally made by the North Eastern Railway in the early part of the 20th Century, to warn drivers of those new-fangled motor cars that the bridge they were about to cross had a limited weight capacity. The sign itself refers to two Acts of Parliament: the Motor Car Acts of 1895 and 1903.

Signs of this time were replaced with standardised signs over the course of the next few decades. A small number of signs of this kind survive, such as this example. It can be found on a bridge on the approach road to Goathland Station, on the preserved North Yorkshire Moors Railway.

I have made a virtual model of this sign for the Trainz Railway Simulator program (a picture of the model can be seen here), and it should be available for downloading by registered users of that program in the near future.