Let me start with: Riding without a helmet is dumb. But then again, so is riding.
So what are the chances regarding the stupidity of both? Well, riding increases our risk of dying by 3500%
(rate per vehicle miles traveled) over your risk in the average cage.
So what about helmets? Well, they reduce risk in the following ways:
Non-DOT non-full-face: Reduces head injuries, but does not protect against face or brain injuries.
DOT non-full-face: Reduces head and brain injuries, but does not protect against facial injuries.
DOT full-face: Reduces head, face, and brain injuries.
So let's look at these various injuries...
Of all accidents assessed and recorded at the scene (which, by definition, ignores accidents where the motorcyclist was well enough to get away without emergency services arriving in time) we see:
Unhelmeted riders received moderate to severe facial injuries 6.6% of the time. Helmeted riders (both DOT and non-DOT) received moderate to severe facial injuries 5.1% of the time.
This is from a sample of 101,987 riders (CODES 2003-2005). Of these 43.1% were unhelmeted, 48% were DOT-helmeted, and 8.9% were non-DOT helmeted (NOPUS). The difference between the two groups likely being that unhelmeted riders, by definition, don't have face protection, but the helmeted group includes full-face helmets.
From the same sample, unhelmeted riders received head injuries 8.1% of the time. Helmeted riders (both DOT and non-DOT) received head injuries 5.3% of the time
But rarely do head or facial injuries alone result in death. Disfiguration, sure. But death usually only comes in the form of infection when it comes to these types of injuries. That would be why in 2003-2005, only two motorcyclists died due to head or facial injuries (both of whom, ironically, were wearing helmets), which brings us to the best reason for DOT-compliant helmets:
Traumatic Brain Injury (TBI)
From the same group as above, 8.7% of unhelmeted riders received TBIs, and 6.3% of helmeted riders did as well
. If we assume that non-DOT helmets do nothing to reduce the risk of TBI over being unhelmeted, and using the NOPUS finding that approximately 84% of these helmets are DOT-compliant, we can extrapolate to a reasonable degree of certainty that the change from 8.7% to 6.3% (a 2.4% drop) would instead be a change to 5.8% (a drop of 2.9%) if all of the helmets worn were DOT-compliant
So what does this tell us? That wearing a DOT-compliant helmet saves you from TBI in 2.9% of wrecks (8.7% - 5.8%), and does nothing for you in the other 97.1% of wrecks, where either your head didn't strike anything hard enough to matter, or struck it so hard that even a DOT helmet didn't prevent death or a TBI. This is likely why so many people who have wrecked without a DOT helmet think that they didn't need one when, in fact, they just weren't part of the unlucky ~3%.
So what about levels
of brain injury?
Of the 101,987, there were 1,623 potential TBI, 3,378 mild/moderate, and 2,522 severe, broken down as follows for without/with helmet:
None: 40,138/54,326 (91.3% / 93.7%)
Potential: 912/711 (2% / 1.2%)
Mild/moderate: 1,607/1,771 (3.7% / 3.1%)
Severe: 1,326/1,196 (3% / 2.1%)
So what we can see here is a slight reduction in all three categories of TBI based on helmet use. Potential, M/M, and Severe are reduced by 0.8%, 0.6%, and 0.9% of the overall total respectively for their categories. If we again assume that 84% of the helmeted riders were DOT (per NOPUS) and that non-DOT helmets do nothing to prevent TBI, then we can again extrapolate expected decreases of 0.9%, 0.7%, and 1.1% for these same categories had all of the helmeted riders worn DOT-compliant helmets.
In short, this means that DOT helmets do reduce TBI in each category in roughly 0.9% of cases. It also means that this reduction is greater than the number of those shifted from the "dead" to the "alive but brain-injured" category.
As for death, ~3,750 died. If we use the NHTSA's estimate that helmets are 37% effective in saving lives, we can calculate as follows:
48% wear DOT helmets, therefore 48,954 of the 101,987 would have been wearing one at the time of their wreck.
100% of those not wearing DOT helmets did not derive this "37%" benefit and therefore died.
63% of those wearing DOT helmets did not derive this "37%" benefit and therefore died.
37% of the 48% entering into accidents which would kill an unhelmeted rider live. (17.76% of the total entering)
63% of the 48% entering into accidents which would kill an unhelmeted rider still die. (30.24% of the total entering)
100% of the 52% entering into accidents which would kill an unhelmeted rider die anyway. (52% of the total entering)
Ergo: 52% plus 30.24% = 82.24% of all of those entering into an accident which would kill an unhelmeted rider died. 17.76% lived because of their DOT helmets.
Since we know that 3750 died, this means that 3750 is 82.24% of 4,560. Thus 4,560 are the total number who entered into the accident that would have killed an unhelmeted rider.
Thus, 810 lives were saved in this case. If all of these 4560 riders had been wearing DOT helmets, then another 877 of the 2,371 non-DOT-wearers would have survived.
So what are the rates regarding fatalities here?
2,371 non-DOT-wearers died, which is 4.5% of all non-DOT-wearers who wrecked and were part of the study.
1,379 DOT-wearers died, which is 2.8% of all DOT-wearers who wrecked and were part of the study.
Wearing a DOT helmet does the following
Saves your life in about 1.7% of accidents
Prevents traumatic brain injuries in about 2.9% of accidents
Wearing any helmet does the following
Prevents head injuries in about 2.8% of accidents
Prevents facial injuries in about 1.5% of accidents
: It's odd to get on someone about something that increases their risk of death in a wreck by 1.7%, but be okay with doing something else that increases it by 3,500%.