17 Parking and supervision of vehicles containing dangerous substances
1 The driver of a road tanker or of any other vehicle carrying a tank container shall (when this paragraph applies) ensure that the vehicle concerned is parked in a safe place when it is not being driven, except—
(a) when supervised at all times by him or by a competent person over the age of 18 years; or
(b) in circumstances where no such competent person is present, when the vehicle has been damaged or has broken down on a road and the driver has left the vehicle to seek assistance.
2 Paragraph (1) shall apply where the road tanker or tank container is required by these Regulations to display hazard warning panels and the emergency action code (if any) required to be shown on those panels pursuant to Schedule 3 ends with the letter “E”, except that that paragraph shall not apply where—
(a) any tank or compartment of a tank which formerly contained a substance which has an emergency action code ending with the letter “E” is nominally empty;
(b) in the case where the substance identification number 1270 is displayed on the hazard warning panels concerned, no petrol is being carried or any tank or compartment of a tank which formerly contained petrol is nominally empty; or
(c) in the case where the substance identification number 1268 is displayed on those hazard warning panels, no toluene or petroleum distillate having a flash point of less than 21°C is being carried or an! tank or compartment of a tank which formerly contained any such substance is nominally empty;
and in this paragraph “nominally empty” means, in relation to a tank or compartment of a tank, that as much of the dangerous substance which it contained as it was reasonably practicable to discharge from it has been so discharged.