Where dangerous goods, within the scope of the regulations, are being carried on a goods vehicle, the markings that you see will be dependant upon the type of goods carried, type of vehicle and type of journey.

All vehicles, except unaccompanied trailers and single substance tankers on international journeys, carrying dangerous goods on either national or international journeys must carry an orange board at the front. This board must have a black border and measure 40Omm wide by 300mm high. The size is reduced for smaller vehicles and containers.

The Carriage of Dangerous Goods and Use of Transportable Pressure Equipment Regulations 2004 [CDG etc. Regs 2004]  lists many disapplications and exemptions from the regulations.  Perhaps the biggest is that they only apply to vehicles carrying dangerous goods in the course of work.  The full list is available here.

Vehicles carrying packaged goods only:

Vehicles that are carrying packaged goods that come within the scope of transport categories 0 to 4 must be labelled with an orange board to the front and an orange board to the rear. The dimensions are the same as those set out above.

This is compulsory on goods vehicles of any size that come within the scope of the regulations.

Where the quantity of goods carried falls below the criteria laid out in transport categories 0 to 4 the display of these orange boards becomes optional.

Tankers and tank containers carrying dangerous goods in any quantity must display the following:

The orange panel described previously:

An orange panel divided vertically in to two halves. The top contains the Hazchem Emergency Action Code. The bottom contains the United Nations four-figure substance identification number.

The purpose of the code is to give immediate information to the emergency services on the action to be taken at the scene of an incident involving dangerous goods.

The example above is seen quite commonly on petrol tankers.