Though bucking royal protocol has become something of a trend since Meghan Markle joined the family last spring, some rules are to be taken more seriously than others.

In the spirit of the holiday season, the Cambridge clan all rode in the car together to the Queen’s pre-Christmas lunch on Wednesday, with Prince William and Kate Middleton in the front seats and Prince George and Princess Charlotte in the back.

The British Royal Family Arrive At Buckingham Palace For Christmas Lunch
Credit: Neil Mockford/Getty Images

And though this is super sweet and makes me want to write a think piece titled “The Cambridges: They’re Just Like Us!,” it’s actually a pretty serious violation of royal policy.

Since Prince William, Prince George, and Princess Charlotte are all heirs to the throne, they’re technically not supposed to travel together in case of an accident. It tends to apply more strictly to air travel, but considering the late Princess Diana’s tragic fate, automobile travel is surely taken into account.

Royal etiquette expert Alexandra Messervy of the The English Manner concurs, telling InStyle via email that though rules are definitely more strict for air travel, they still need to consider Queen E's permission for car travel. "Nowadays, it would appear that [traveling together] by car doesn’t carry the same issue [as by plane], but they will of course have security police protection," she wrote, noting that police protection wouldn't help in the event of a road accident.

"But I suppose they believe that things are relatively safe now," she adds of why they're granted permission to travel by car so frequently.

According to Business Insider, the rule is more lax when it comes to small children — Will and Kate already broke it when they flew to Australia with Prince George — but once the children are 12, they will be more likely to travel separately, or get approval from the Queen herself.

2016 Royal Tour To Canada Of The Duke And Duchess Of Cambridge - Victoria
Credit: Chris Jackson/Getty Images

Drive safe, William!