One option would be to formalise the instructions signs in TSRGD as endorseable, which would then make it an offence to contravene them. These could then be dealt with by a fixed penalty notice for 3 points and a £100 fine or it could be referred to court where the fines can be higher, and discretionary disqualification may be given.
Another would be to make it a specific road traffic offence.
Another would be to make it a standalone offence elsewhere and specify the penalty.
In practice I doubt it would make any difference. People who leave the gates open likely aren't concerned that it's an offence, because after all it already is one (and it says so on the signs). And I think a £1,000 fine would already be enough to put people off if anything was going to. People leaving them open are either ignorant, lazy, or thinking to themselves oh I'm only going to be at the house 2 minutes doing this delivery, I'll close them on the way back.
As with many crimes, the risk of getting caught is a more effective psychological tool than the penalty for having been caught. To that end, rather than trying to change legislation, which takes an age and is subject to all sorts of objections and time wasting and likely wouldn't make much difference, Network Rail would probably be better off identifying the crossings where multiple reports have come in about gates being left open and installing CCTV, possibly with ANPR, and possibly with some AI software which proactively alerts a control room about gates being left open. I suspect this would have a greater impact on offending rates, even if the first action against a particular driver is just a warning letter to educate them on the rules and that enforcement actions will be taken if they repeat the action.
