Article 17

1. With a view to ensuring a homo-geneous system, the road signs and signals adopted in each Contracting State shall, as far as possible, be the only ones to be placed on the roads of that State. Should it be necessary to introduce any new sign, the shape, colour and type of symbol employed shall conform with the system in use in that State.
2. The number of approved signs shall be limited to such as may be strictly necessary. They shall be placed only at points where they are essential.
3. The danger signs shall be placed at a sufficient distance from the object
indicated to give road users adequate warning.
4. The affixing to an approved sign of any notice not related to the purpose of such sign and liable to obscure it or to interfere with its character shall be prohibited.
5. All boards and notices which might be confused with the approved signs or make them more difficult to read shall be prohibited.

 Convention of Road Traffic signed at Geneva September, 19 1949 (English)