If you have a modular or flip-up helmet warm weather can tempt you into riding flipped up, as it does most of the Metropolitan Police in the summer. But is it legal?
The answer is that it is not illegal, and this is not me taking a jurisprudential point. In English and Welsh law, which is a common law system, everything is legal unless it is made unlawful. Because Road Traffic law is governed entirely by Parliament the law is contained within statute. Statute is interpreted strictly and, this may come as a surprise, in favour of the accused. If you want to delve into the law it is contained in a short 1998 Statutory Regulation.
The tests can be summarised as:
(i) Is the helmet type approved to International or British standards?
(ii)Does it have a strap?
The key test is ECE 22.05 which is an international UN treaty, to which the UK subscribes as a member of the EU. Even if Brexit results in a complete crash out of the EU and all European treaties, your helmet won’t become illegal because the 1998 regulations stand alone in UK domestic law and your helmet will remain internationally legal under ECE22.05.
So on those rules, riding flipped up does not breach the law, if your helmet is British Standard 1869:1960 or ECE 22.05 compliant, but riding unstrapped does. BS alone would be quite scary as it has not been updated since 1975.
So, in short. If you want to ride flipped-up the criminal law does not stop you. However, there are potential legal consequences. If you ride flipped-up and you suffer an injury or an exacerbation of an injury directly caused by being flipped-up (the obvious risk being to your jaw, but less obviously rotational forces generated to the neck through the raised chin piece) It could be argued, with some force, that your use of the helmet could amount to contributory negligence.
There is no decided law on that point. There is a general acceptance, supported by precedent in England that riding with your strap undone reduces your claim by some 10% If you sustain a head injury, and riding with your strap undone is a criminal offence. So riding flipped-up could fall within contributory negligence, but I am far from convinced.
If you really want to have lovely airflow on a hot ride I would go open face, possibly using a giant hipster beard as impact protection: an open faced helmet could not constitute contributory negligence any more than the driver of an open topped car being hurt if their car rolled could constitute contributory negligence.
Finally there are now ‘urban’ helmets which are approved, internationally, with the chin piece down or up. Riding flipped up with one of these would not create legal difficulties.