Photo above is my full-face helmet that I was using when I had a small motorbike accident a few years back. As you can see I was lucky to be using a helmet and to be more precisely to be using a full-face helmet. Because not using a full-face helmet would have had a dramatic impact on my face.
Since then it became a personal thing to tell people about using a helmet. If you are going to ride a motorbike or scooter (by law the same in Thailand) then ALWAYS use a helmet and if you are riding regularly use a full-face helmet. Mind you I also say this to friends who are happily riding around without helmet. And I will say it again and again.
It does not matter fast or slow, short or long .... an accident can happen any time. So I ALWAYS use a full-face helmet, leather cloves, a long trouser, good shoes and at least a jeans-jacket. For longer rides I will ALWAYS be wearing motor-boots, leather jacket, motor-gloves and my full-face helmet of course. For multiple-day tours I will also be wearing my motor-trouser with kevlar.
So yes any tourist renting a scooter in Thailand now thinks 'Are you nuts, it is bloody hot" and "It is only a scooter" or "I will be careful and ride slow". That all does not matter, look to the statistics of motor / scooter accidents in Thailand. Look to the roads, look to the others on the road.
So when you are going to rent a scooter for a longer time in Thailand just buy your own helmet at BigC or Tesco-Lotus. And ALWAYS use it! Then when riding wear shoes instead of flip-flops, wear a trouser instead of a short or bikini. And wear at least a light jacket. And if you are planning to go touring with your scooter buy also some gloves. And always use your helmet with a screen, so your eyes are protected from all the dirt on the road.
Happy scooter or motorbike riding in Thailand!