aspects where the context allows the invention will now be described in more detail without limitation with reference to the accompanying figures as can be seen from fig1 the device in one embodiment comprises two discrete elements part a being a base element housing and helmet engaging cable and part b being the discrete locking element part a comprises an essentially circular cable housing 1 which accommodates a cable roll or reel 2 and which together with a bottom part 3 and circular clamp 4 form a base element and which reel 2 carries a retractable plastic coated cable 11 having a metal core the cable 11 which may be similar or identical to the one used for securing the bicycle itself to a parking facility is wound up on a spring loaded mechanism accommodated in the bottom part 3 of the cable housing 1 so it can be pulled out from the latter and it returns to it automatically the cable housing 1 is closed at the bottom with the aid of a bottom part 3 part a also comprises a circular clamp 4 for fixing the cable housing 1 to the handlebar of a bicycle with the aid of a fixed undetachable bolt 5 these three components 1 2 3 4 and 5 may form a single moulded unit which is fixed to the handlebar by the user and which remains there for successive use part a also comprises in the respective embodiments illustrated in fig1 and 2 a pull tab 6.