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Policy statement
The Fatality Analysis Reporting System (FARS) database brings together information about fatal traffic casualties in the USA. Since 1994 the database has included information about bicycle helmet use.
Like most other data elements reported under FARS, data on helmet use is derived from individual Police Accident Reports (PARs) collected by the various state agencies that deal with traffic crashes. Since data on bicycle helmet use is not considered a particularly high priority in most jurisdictions, relatively few state PAR forms have the type of simple check-off box commonly associated with seat belts or motorcycle helmets (e.g. 'used', 'not used', or 'unknown') that allow for easy transcription into the FARS database. In these cases (i.e. the vast majority involving bicyclists), any information on bicycle helmet use must be obtained from the narrative of the crash prepared by the attending police officer. If no definite mention is made as to whether a bicycle helmet was used or not (which is still all too common due to the relatively low priority in determining actual helmet use among involved bicyclists), such cases should be recorded as 'unknown' according to the FARS coding protocols. Unfortunately, it appears that nearly all of these cases that should have been coded as 'unknown' (including a considerable number where the bicyclist actually was using a helmet, but such usage was either never noted or overlooked in the narrative) were instead coded as 'not used' - particularly in the first years of helmet data collection, 1994-98.
One strong indicator that the FARS bicycle helmet use data should not be fully trusted is the fact that the 'unknowns' are so few in number. It is simply not credible that a low priority data element such as bicycle helmet use would have a precision associated with it that is a factor of 20 better than that seen for much higher priority data elements such as seat belt or motorcycle helmet use (0.5% 'unknowns' vs. 11% or 10%). Even more persuasive is a direct comparison of FARS data with equivalent state data. Though very few states make any real effort to determine bicycle helmet use in their annual traffic crash summary reports, two that have done so for at least a decade, California and Florida, together account for approximately 30% of all US bicycle fatalities.
California data from the StateWide Integrated Traffic Records System (SWITRS) indicates that at least 13.2% of fatally injured bicyclists were using a helmet during the period 1994-98 (since SWITRS combines 'unknowns' and 'not used' into a single category, the helmet use rate is actually a minimum estimate and could be much higher, depending upon the relative number of true 'unknowns' and how biased the distribution might be) (Cal Highway Patrol), but only 3.4% supposedly were doing so according to FARS, 1. Likewise, Florida data from the Department of Highway Safety and Motor Vehicles (DHSMV) indicates that 6.5% of their fatally injured bicyclists were using a helmet during the same period (disregarding 'unknowns') (Florida HSMV), but less than 0.2% (i.e. just 1 out of nearly 600) according to FARS, 1. And while the reliability of bicycle helmet use data in FARS has clearly improved in recent years (>18.7% helmet use from SWITRS vs. 14.0% FARS for 2001-04 CA data, and 6.2% DHSMV vs. 4.1% for FL data from the same period), it is clear that FARS continues to undercount such use in too many cases.
Though overall bicycle helmet use in FARS has reached approx. 10% for the period 2001-04, a number of states within FARS now routinely record use rates of 20% or more (CO, GA, HI, ID, ME, NV, OK, WA), and a few actually record use rates in excess of 40% (MA, NE, VT, WY). An even larger number of states continue to register helmet use rates of 5% or less (AK, DE, IN, KS, MI, MS, MO, MT, NH, NM, NC, ND, RI, SC, SD, UT, VA, in addition to FL), and the skewed nature of the distribution (median = 7.7%) strongly suggests that the overall helmet use rate among fatally injured bicyclists is more likely now to be in the 20 - 25% range nationally, rather than the 10% indicated by FARS. In addition, a small but non-trivial number of cases are now clearly being miscoded by FARS, with about 2% of fatally injured bicyclists during 2001-04 supposedly using some kind of seat belt or child's safety seat (which should , of course, apply only to occupants of motor vehicles) instead of a helmet - up from zero prior to 2000.
Even a cursory examination of the data indicates FARS was underestimating actual helmet use among fatally injured bicyclists by up to an order of magnitude or more during the period 1994- 98. Although the situation has improved considerably since then, FARS continues to underestimate overall bicycle helmet use in the US by a factor of two or more as of 2004 (the most recent data available) (FARS, 1).
State Wide Integrated Traffic Records System. California Highway Patrol. 
Fatality Analysis Reporting System web-based encyclopedia. National Center for Statistics and Analysis.
Florida traffic crash facts. Florida Department of Highway Safety & Motor Vehicles.