My suggestion on this case is to draft a company policy as the best way to encourage your employees to wear helmets through company policy. Before sending a circular to your employees, please draft a policy stating that wearing a helmet is mandatory while driving a two-wheeler (only inside the company premises). Your policy must highlight the local regulation that states wearing a helmet is mandatory. We can advise our employees to use helmets outside the company premises as well, but if they fail to follow this, it is up to them.
In this case, if I were in your place, I would invite all the people/employees to a weekend party where they should attend with their families (married individuals should attend with their spouses, unmarried individuals should attend with their parents, and I highly suggest their mothers attend). Begin the party by thanking all the employees for their support, and then announce the new policy regarding helmets. Explain the reason briefly (using a PowerPoint presentation can add value) and request the spouses/parents to always remind their sons/daughters/wives/husbands to use helmets when leaving home. This way, they can reinforce the habit at home, which is the best way to implement these practices into daily life.
Note: The implementation may vary depending on the number of employees in your company.
Please check the CITEHR FIND INFO TAB for a lot of information, such as the kinds of issues you may face after sending a circular, as everyone tends to have reasons for everything.
Please see the attachments; I hope it helps. (I downloaded the attached PowerPoint presentation from here as well.)
Thank you.