There are, of course, some limitations to the data.
- The data appears to be solely from MPD, but as noted in a 2007 report on Traffic Safety Information "In the District of Columbia, the DC Metropolitan Police Department (MPD) and other law enforcement agencies such as the U.S. Capitol Police, U.S. Park Police, Secret Service, etc., obtain crash information. The MPD and some of the other agencies record the crash data onsite in a paper form using a Traffic Accident Report (PD-10, see Appendix C). Although some agencies apart from MPD use the PD-10, there is no uniform crash reporting form used by all local enforcement agencies throughout the District."
- It only uses data for which a PD-10 crash report is written. Not all crashes get reported to the police, and not every crash reported to the police gets a PD-10
- The PD-10 needs to be revised. In the same 2007 report quoted above "The current version of the PD-10 was designed in 1986 and lacks conformance to the Model Minimum Uniform Crash Criteria (MMUCC) and the Manual on Classification of Motor Vehicle Traffic Accidents (ANSI D16-1), as well as SAFETYNET data element definitions. This makes it difficult, for example, to report motor carrier crash data to the Federal Motor Carrier Safety Administration (FMCSA). Further affecting the adequacy of reporting to FMCSA is the lack of reporting of nonfatal, nonserious commercial vehicle crashes by the MPD." But it's unclear that the form was revised, since in 2012, they were still talking about a forthcoming revision and elsewhere saying that they would take a recommendation to add a bicycle-specific line "into account in any future revisisons." More on the PD-10 and its flaws is here. Here is what I think the PD-10 still looks like. The 2007 form is much better (as it allows the person filling it in to choose "bicyclist" for any one of the involved people).
So we're really looking at a subset of all bicycle crashes, based on information that is filled out in a flawed form, by people who are busy dealing with other pressing issues like injured people, traffic disruptions etc...
Oddly, 2007 report mentions all the data - the Model Minimum Uniform Crash Criteria (MMUCC) - that the NHTSA requests from the states
DDOT reports the following for the information it gives
Person Type: Bicyclists - FULL
Bikeway - NONE
Helmet use - FULL
Helmet is liste on the PD-10 under the seatbelt/bag code section, but as reported elsewhere, this is massively under-reported and mis-reported.