At first the sign seemed an odd thing to see displayed so blatently in a vehicle, but then I realised that the van would be owned by the young men's employer, and that it was therefore effectively a part of their workplace - hence the sign.
This got me thinking about an idea that has been floating quietly around the road safety world for a while now, which is this: for millions of people the road is a place of work, but when it comes to the safety of these workers, all the usual regulations are suspended.
If you work in a hazardous place or with dangerous machinery, your employer must go to great lengths to ensure you cannot come to harm. As a student I once spent a summer working a powerful hydraulic press - a brief lapse of attention, which was entirely probable given the repetitive nature of the task, would have meant instantly losing a hand. As a result, the press was designed so that it would only operate if two widely separated buttons, well away from the jaws of death, were pressed simultaneously and a protective screen had already been lowered. This ensured that the operator's hands were nowhere near the danger area when the machine went to work. In other words, no matter how careless the operator was, the workplace was explicitly designed so they simply could not come to misfortune.
Now consider the person who drives as part of their job. Where are that person's protections and failsafes? Unlike the press operator, the sales rep or courier is frequently working in an environment where a moment's lapse of concentration could well kill them, or someone else, but nothing at all is done about it. The employer sends their workers out into a hazardous environment but has no responsibility to ameliorate the risk to their employees, and is not liable when one of their employees is killed or injured in the course of their work. The owner of the white van I saw didn't even have to ensure their employees were wearing seatbelts, for goodness sake! I don't think employers are required even to do something as basic as telling their employees not to speed. The working environment of anybody who drives professionally is practically Dickensian.
Of course, in certain sectors some measure of protective legislation does exist: there are a handful of rules on how many hours a long-distance lorry driver can travel without a break, for example. But there are still millions of people who drive as part of their work - everyday ordinary people like postal workers, couriers, midwives, photocopier repairers, consultants - who are regularly put in danger without any protection. Unless we're talking about protection from making an adult decision to smoke some legally obtained and taxed tobacco, in which case don't worry: the government is all over that one.