On the public highway in the UK this is illegal and subject to a fine, depending on the local authority. In others, such as the Tesco car park (private land and so only subject to their own rules/enforcement) it is not - we just have to walk the length of the car park when, in many cases, this is painful and dangerous.
This side of the card will face outwards and be visible to passers by:
This side of the card will face in to the driver and so be more likely to survive wet weather:
Feel free to use these web sized JPEG files if you are in the UK and *do* have a valid blue card of if you live elsewhere and they might come in useful to you. I have the originals as .PSD (Photoshop) files and can always create higher resolution files or different wordings for other countries if people would like me to.
P.S. My hubby wants me to print them at A4/letter size to better shame the drivers because passers-by will see them better!