The bicycle helmets are almost just made by styrofoam and many people use it instead of motorcycle helmet with full protect and plastic,metal parts.the truck can easily crush it and almost helmet are useless,but when we crash the head to the road,a good helmet will slide out and absorb the force that can help us from being broken the skull while a foam helmet just brake out or transmit the energy to our head and spinal.
Wearing a bicycle helmet is safer than not wearing one. Bear in mind that a helmet that has been "crashed" needs to be replaced. Its ability to absorb/diffuse energy after having been landed on will be degraded.
Personally I never wear a cycle helmet because I find them uncomfortable and I feel that they do impair my ability to be aware of the environment around me. Nonetheless, it is obvious to me that I am putting myself at risk of death or serious injury in certain types of accident where a helmet-wearing cyclist will walk away virtually unscathed.
I know someone who DID have their head run over by a truck whilst wearing a helmet – after much medical care they were fully rehabilitated. I doubt I would have survived the same accident. I expect my organs might have been useful (maybe not the corneas!).
The only way this question can really be answered is with statistics, not opinions. Per 100 cyclists, how many were killed/hospitalised BEFORE helmets became the norm, and how many now? Even then, there are many, many complications, including, but not limited to 1) quantity/type of traffic on the roads 2) quality of brakes on bikes 3) design of road vehicles 4) road design/construction standards 5) type of riding being done 6) age/fitness demographic of typical riders 7) relative availability of cycle paths.
With every type of safety improvement, there are some losers and there are more winners. I know someone who swears that his life was saved by NOT wearing a seat-belt. A steel beam went through the windscreen and through his seat. Luckily he wasn’t in it at the time because he’d been thrown sideways across the passenger seat since he wasn’t retrained by the seat-belt. You’ll always hear these kind of stories, but only the numbers can really tell us whether seat-belts are a POSITIVE or a NEGATIVE overall, and that’s a no-brainer (no pun intended).