It seems that people who rent bikes don’t tend to wear safety helmets.
Today’s invention is therefore a bicycle helmet vending machine that would sit alongside a rank of rentable citybikes.
There would be a number of oval recesses on the machine’s front face into which one would try to place the crown of one’s head. Having found one that fitted, the user would pay and receive a helmet in the right size (these would be stacked, nested tightly together inside).
The helmet would have bonded to its strap a key which would then be used to release the lock on a bike.
Locking the bike at some intermediate stop would require the same key to be used, so that the helmet would need to accompany the rider -and thus be much more likely to be worn.
Failure to return the helmet to a machine would result in an extra charge made to the credit card used to rent the bike.