Thursday, December 05, 2013

San Francisco systematically under-reports cycling accidents

trauma room

Since it's behind a paywall, below is my transcription of the important study on bicycle injuries in San Francisco I wrote about here and here. The New York Times referred to the study in a story in October. 

I left out only tables, figures, and the endnotes. Emphasis added.

Dahianna S. Lopez, RN, MS, MPH, Dharma B. Sunjaya, BS, Shirley Chan, Sarah Dobbins, MPH, and Rochelle A. Dicker, MD, San Francisco, California

Bicycling and walking have become increasingly important modes of transportation in urban environments. The popularity of biking has grown in the last decade for various environmental, economic, and public health reasons, including the challenges posed by the cost of fuel, threat of greenhouse emissions, and diseases such as obesity, diabetes, and heart disease. The transportation, public health, and urban planning communities have come to view active transportation as a critical part of the solution to these challenges.

As we collectively work to strengthen sustainability in transportation, we must pay particular attention to the health and safety of those who use these modes of transport. Injuries from bicycle crashes are not only potentially life altering or deadly for individuals but are also costly. In an analysis of the medical and social consequences of bicycle injury, it was shown that hospitalized bicyclists experienced issues including persistent disability, cognitive and behavioral changes, and permanent work disability. To continually improve the safety and accessibility of alternative transportation for all communities, there must be a systematic way to view and track accurate surveillance data.

According to the National Highway Traffic Safety Administration, in the United States in 2009, 630 bicyclists were killed and 51,000 were injured after colliding with a vehicle. These, like many crash statistics, rely primarily on police reports and are typically underestimated. For example, in Germany, the number of bicycle crashes determined by hospital data exceeded the number of crashes reported to police by nearly two times.

Underestimation of crash rates results partly from a lack of recorded data on cyclist-only (CO) injury crashes, that is, crashes in which no contact is made with an automobile. Moreover, there is bias in both police-reported and hospital-reported bicycle crash data because less severe injuries are reported and treated less frequently. In an attempt to elucidate data on CO crashes, a study conducted in New Zealand reported that 22% of CO injuries could be linked to police reports, compared with 54% of injuries resulting from an auto-versus-bicycle (AVB) collision.

No study to date has compared CO injuries to AVB injuries with regard to their health outcomes and direct medical costs in the United States. The overall goal of this study is to better identify CO injuries and understand the severity and cost of bicycle injuries in San Francisco. We hypothesized that CO injuries carry an equal or greater burden of injury per patient and greater overall cost when compared with AVB injuries. 

This article addresses the following study aims: (1) test the hypothesis that CO injuries are underreported in police records compared with AVB injuries; (2) compare CO and AVB injuries in their incidence, injury severity, and admission rate; and, (3) report the medical cost of bicycle-related injuries from our trauma center database.

Study Design and Population
We conducted a 10-year retrospective cohort study of AVB and CO crash injuries treated at San Francisco General Hospital, an urban Level I trauma center, between January of 2000 and December of 2009. San Francisco General Hospital (SFGH) is the only trauma center for the city and county of San Francisco. The University of California San Francisco's Institutional Review Board granted our group permission to conduct this research study.

Data Sources
We used injury E-codes from the DRG International Classification of Diseases---9th Rev. (ICD-9) to select bicycle-related injuries from the hospital's trauma registry. We selected individuals with injuries who were either treated in the emergency department and released within 24 hours or admitted to the hospital. Using the San Francisco Department of Public Health Billing Information System, we collected hospital charges, professional fee charges, and payments for all patients. 

Police records were drawn from the Statewide Integrated Traffic Records System (SWITRS). SWITRS is a database that serves as a means to collect and process data gathered from a collision scene and is accessible by the public. Patients injured outside the city and county of San Francisco were excluded from the sample...

Police- and Hospital-Reported Bicycle Injuries
Police reported 3,717 bicycle injuries from 2000 to 2009; SFGH reported 2,504 bicycle injuries from 2000 to 2009...

Match between Police and SFGH Data
After matching the SFGH to SWITRS data using a probabilistic linking approach, we identified three categories : matched data, SFGH-only data, and SWITRS-only data. Thirty-one percent of SWITRS data were matched, and 45% of SFGH data was matched. The remaining 55% of bicycle injuries treated at SFGH were not associated with a police report...

A more detailed analysis of the matched data from 2000 to 2009 showed that 90.9% (1,033) of the cases were AVB injuries and 9.1% (104) were CO injuries. Of all injuries identified at SFGH (2,504), 41.5% were AVB injuries and 58.5% were CO injuries. Hence, among matched data, we saw substantially fewer CO injuries (9.1% vs. 41.5%), suggesting that CO injuries may be underreported in police data. Our data also showed that crashes with severe injuries were matched more frequently than crashes with minor injuries...

Patient Outcomes from SFGH-Identified Injuries
SFGH-Identified Bicycle-Associated Injuries
Of all injuries treated in the hospital from 2000 until 2009 (2,504), 37% arrived by ambulance and 3.2% self-transported to the hospital. Transport data were missing for the remaining population. Helmet use data were available for 1,178 bicycle associated injuries. Among those, 80.8% of injured patients were not wearing a helmet at the time of the injury. In addition, 28.2% of injured patients required admission to the hospital...

AVB versus CO Injuries
Those who had CO injuries were three times more likely to have worn a helmet at the time of injury...Nineteen percent of AVB injuries and 53% of CO injuries required admission to the hospital intensive care unit. Based on SFGH data, we found that those with CO injuries were four times as likely to require intensive care unit admission...

The median number of hospital days was 3 days for both admitted AVB and CO injury patients. The median Injury Severity Score for both AVB and CO injuries was 10, but a higher proportion of AVB victims died compared with CO victims (3.7% vs. 2.0%). This distribution however was not statistically significant. Among injuries for which we had hospital disposition data (39% of the total), there was no difference between those with AVB and CO injuries.

Cost of Bicycle Injury in the City and County of San Francisco
After adjusting for economic inflation, the total cost for bicycle-related injuries treated in the hospital in 10 years was $36.4 million (expressed in 2009 dollars). Admitted patient costs accounted for 77% of the total cost of injury. In addition, 74% of the total cost of injury was charged to public funds, including Medicaid and Medi-Cal. Eight million dollars are omitted from this calculation given that the insurance status variable was incomplete for many non-admitted patients.

From 2000 to 2009, the cost of injury per year more than doubled, from $2.6 million in 2000 to $6.8 million in 2009. Across all years, the median injury costs for admitted and non-admitted incidents were $19,207 and $4,088, respectively.


Surveillance Gaps in Bicycle Injury Data
We found that police-generated bicycle injury data underreport injuries. Furthermore, this underreporting was much more pronounced for CO[cyclist-only] than for AVB[auto-versus-bicycle] injuries. Our data showed a higher proportion of matched data from the SFGH database compared with the SWITRS match ratio (45% vs. 31%). This suggests that there are fewer CO crashes in which the police are called and the victim went to the hospital. This trend is also demonstrated in the proportions of matched data for SFGH and SWITRS during all years, leading us to assume that not all victims who report the incident to police are then treated at the hospital. This reporting bias is important to consider when looking at injury severity data.

Injury Severity
In our comparison of AVB and CO injuries, we found that CO injuries four times more likely to be admitted to the hospital after presentation in the emergency department. Despite this increased likelihood of admission, our data indicated that length of hospital stay, hospital disposition, and the Mean Injury Severity Scores were not different among AVB injury patients and CO injury patients.

In addition, we found notable data on helmet use among those injured, which shows that a significantly higher proportion of AVB injury patients report not wearing a helmet. Owing to limitations in the retrospective data, we were not able to fully explore the scope of bicycle injury in San Francisco. Further studies are necessary to understand the differences in injury severity, predictors of injury, comorbidities, the role of alcohol use, particular demographics of bicycle riders, and the sequelae of AVB and CO bicycle injuries.

Costs of Bicycle Injury
In reporting the medical cost of bicycle related injuries, we found that AVB patients incurred a significantly higher cost of treatment than do CO patients. The cost for both AVB and CO grew tremendously from 2000 to 2009, nearly tripling for cost of AVB medical care and doubling for cost of CO medical patient care. Of note, we found that 74% of the cost of injury was charged to public funds. Given this information, it is clear that improved bicycle injury prevention efforts may result in reduced state and federal health care expenditures.

Traditionally, SWITRS data are used as the criterion standard to inform policy and set priorities for urban projects involving the built environment and other traffic safety measures. However, SWITRS data, as previously reported by Sciortino et al, are only one piece of the surveillance puzzle. Trauma centers can play a key role in future collaborations to define issues and develop prevention strategies for cyclist crashes as well as pedestrian crashes.

In this work, we also unroofed a previously underappreciated population of cyclist victims, those who are injured in the apparent absence of involvement with an automobile. Further work needs to be done to understand risks involved in these incidents and to characterize those who are most often involved. Possible issues may include crashes to avoid cars, tires caught in light rail tracks, swerving to avoid pedestrians, or avoidance of unforeseen road hazards.

Creating a surveillance tool that matches data from injured patients who come to the hospital emergency department, ambulance data, and police report data, trauma centers may be able to provide access to information that can improve research and influence city policy and planning initiatives.

Access to bicycling and other nonmotorized modes of transportation is a significant need, especially in light of the public health issues resulting from inactivity. To encourage bicycling, improved roadways and management of risk and risk perception is key. 

Surveillance data from both police records and hospital records will enable a better understanding of the needs for cycling infrastructure and public health education through evidence based research on both national and international scales. By failing to implement effective and sustainable surveillance strategies for these types of injuries, our efforts to prevent bicycle-related injuries may remain limited.

D.S.L., S.C., and R.A.D. designed the study. D.S.L., D.B.S., and S.C. collected the data. D.B.S. and S.D. analyzed the data. S.C. managed the data. S.D. prepared the figures. R.A.D. contributed in the private investigation of the study. D.S.L., S.D., and R.A.D. wrote the manuscript. All authors interpreted the data.

This research was supported by the Centers for Disease Control and Prevention Injury Center Grant R49 CE001178.

Address for reprints: Rochelle A. Dicker, MD, Department of Surgery, Ward 3A, San Francisco General Hospital, 1001 Portrero Ave., San Francisco, CA 94110. email:

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At 6:19 PM, Anonymous Anonymous said...

Thanks so much for highlighting this problem Rob. Fortunately the city is close to building protected bike lanes to reduce the injury and fatality rates of bicyclists and pedestrians. That'll help alleviate this problem.

Keep on it.

At 10:44 PM, Anonymous Anonymous said...

It will be interesting to see how protected bike lanes improve the bike safety situation. Hopefully the doctors involved can do another study in the following years, since fortunately more is being done to make San Francisco's streets more safe for bikes (and safer for pedestrians for that matter).

At 7:33 AM, Anonymous Gregski said...

Anonymous' comment about "protected" bike lanes is a great example of what a hard slog it is for facts when they try to contend with religious faith. The study authoritatively confirms Rob's oft-cited facts that about half of classified bike injuries are "cyclist-only". How does "protecting" a cyclist from traffic prevent injuries that don't involve traffic?

The study concludes that to a great extent we are "flying blind" when it comes to evaluating the injury-cost of various traffic strategies.

As welcome and useful as this study is for providing some heretofore-missing facts, there's an area where we're STILL flying blind: cyclist-cyclist injuries.

Having been injured myself in numerous CC collisions over the past 8 years I am very curious about their prevalence and cost in the general population.

A common-sense hypothesis is that confining cyclists in their own segregated lanes will increase the frequency of CC mayhem, especially given that San Francisco cyclists rival freeway driver in their propensity to crowd across the lane at various speeds.

My impression is that since facts about CC injuries don't exist, our city planners are implicitly concluding that CC injuries themselves don't exist.

At 9:50 AM, Anonymous Anonymous said...

Gregski, excellent point. I suggest multi-lane protected bike lanes. After all, we have multi-lane streets for cars for the same reason - to prevent car-car collisions. Why not the same for bikes?

At 10:15 AM, Blogger Rob Anderson said...

There are a couple of important takeaways from this study: that cycling accidents have always been significantly under-reported by the city, which invalidates how its Collision Reportsand Bicycle Count reports evaluate how safe it is to ride a bike in the city.

This under-reporting is because the city has been relying on police reports of accidents and ignoring records at SF General, the city's primary trauma center.

And the report finds that cyclist-only accidents---aka, "solo falls"---are particularly under-reported, both in the number of such accidents and in their severity.

This finding undermines the premise of the City Hall, Bicycle Coalition anti-car policies---that those "death machines" are the most serious source of cycling accidents, that if the city continues to make it harder to drive our streets will inevitably become safer for cyclists.

Instead the most important conclusion to be drawn from the study: riding a bike in San Francisco---or anywhere, for that matter, according to the CDC---is an inherently risky transportation "mode" and that it's irresponsible of City Hall to encourage people to ride bikes.

At 12:02 PM, Anonymous Anonymous said...

If it's so dangerous because of "solo falls", why do some many people in cities will bicycling infrastructure ride bikes?

At 12:22 PM, Blogger Rob Anderson said...

Because they're gulled into adopting ing the latest green "progressive" fad without understanding how dangerous it can be.

At 12:55 PM, Anonymous Anonymous said...

Because they're gulled into adopting ing the latest green "progressive" fad without understanding how dangerous it can be.

Why am I not dead then?

At 4:06 PM, Anonymous Justin said...

The only real take away from the study is the one that it itself mentions - that hospital records should be used in addition to police records when looking at injury statistics. Fair enough. Although it's interesting to note that the number of injuries reported by the police is higher. Possible explanation - the injuries weren't significant enough to merit going to the hospital.

The only person saying cycling in SF is dangerous is Rob Anderson. The study certainly doesn't conclude it and no where is the word danger used. We can only know if something is dangerous if we know the percentage of injuries of all reported bicycle trips - which we don't. Regardless 3,717 injuries vs. 2,504 injuries over a 10 year period doesn't seem very significant to me. One injury or less a day across the city?

At 1:10 PM, Blogger Rob Anderson said...

Wrong on the study's findings, Justin. The study found that "cyclist-only" accidents were just as serious as the "auto-versus-cyclist" accidents. The SFPD reported 3,717 cycling accidents between 2000 and 2009, but the study found that, by failing to check hospital records, 2,583 accidents were not reported. "Not very significant" when 55% of the accidents are not reported?

The Centers for Disease Control also disagree with your blase attitude. Turns out that just being a passenger in a car is safer than riding a bike:

"While only 1% of all trips taken in the U.S. are by bicycle, bicyclists face a higher risk of crash-related injury and deaths than occupants of motor vehicles do. In 2010 in the U.S., almost 800 bicyclists were killed and there were an estimated 515,000 emergency department visits due to bicycle-related injuries. Data from 2005 show fatal and non-fatal crash-related injuries to bicyclists resulted in lifetime medical costs and productivity losses of $5 billion."

At 4:28 PM, Anonymous Gregski said...

" why do some many people in cities with bicycling infrastructure ride bikes? "

As a high-mileage cyclist myself, my answer is that the benefits of cycling (economy, efficiency, exercise) outweigh the very real risk of physical damage. Damage I have suffered in the past and will suffer again if I continue to ride long enough and far enough.

To encourage bicycling to the inexperienced without advising them about the inevitable and unavoidable crash damage is as irresponsible and immoral as a doctor prescribing a drug to a patient with no disclosure of its known side effects.

At 5:46 PM, Anonymous Anonymous said...

Gregski do you think people who ride bikes don't know that you can fall off?

At 3:03 PM, Anonymous Gregski said...

"Gregski do you think people who ride bikes don't know that you can fall off?"

I don't know. Why are you asking me? I'm a cyclist and you already have my answer on behalf of myself. Why don't you ask other cyclists and tell us what you find out?

On second thought, don't bother telling us because it's irrelevant to this discussion, which is about promoting cycling to the inexperienced and to the voting, taxpaying citizenry with inadequate, and therefore dishonest and misleading, notification of the injury-cost of taking up such a habit.

At 4:54 PM, Blogger Rob Anderson said...

It's the people of San Francisco and prospective cyclists, those City Hall and the Bicycle Coalition are urging to take up cycling, that are not informed about the real dangers of riding a bike in the city.

It's a win-win deal for the city, since it at least marginally helps reduce traffic congestion, but to the unwary citizen who starts riding a bike as a "green" transportation "mode," the dangers are unknown, and the city makes no attempt at telling him/her.

One wonders how much of a genuine oversight this under-reporting by the city really is. How likely is it that the city somehow mistakenly overlooks all the cycling accident victims who show up at the city's primary trauma center, SF General Hospital?

See the Policy Framework of September, 2004, the first volume of the two-volume Bicycle Plan that we litigated over, on page 6-12:

Recommended Action 6.11: "Develop a system for hospitals, emergency rooms, and clinics to report all instances of bicycle injury to the SFPD and to the DPT Bicycle Program Manager." (This was before DPT was merged with the MTA.)

Obviously that simple recommendation was never acted on by City Hall.

On the same page:

"For the last several years, the San Francisco Department of Public Health has been working on an injury data linkage project using hospital admission data. Currently, San Francisco General Hospital is not obligated to report bicycle injuries to the SFPD. This is left up to the injured parties. EMS (ambulance services) is supposed to report bicycle injuries, but many are not reported."

Perhaps City Hall didn't want to publish realistic numbers on cycling accidents, since that would support the idea that riding a bike in San Francisco is actually dangerous, thus undermining the foundation of City Hall's anti-car traffic policies.

At 11:37 AM, Blogger Mario said...


Your argument seems to be that because a lot of bicycle injuries are solo falls, infrastructure is not at fault. But then, the rate of injuries in countries with good bicycle infrastructure are much lower, solo falls or not, so something has to give.

Infrastructure ABSOLUTELY has a lot to do with solo falls as well:

1. Train tracks with wide gaps and lack of proper markings. Competition with cars driving fast forces riders to merge through tracks in a fast and unsafe manner.
2. Riding in gutter is more dangerous due to uneven/slanted surface, debris
3. Riding next to parked cars (due to pressure from motorists) could result in hitting a mirror. It is also unclear if impact with open doors is counted as solo falls or not.
4. Some riders are forced to ride on the sidewalk due to fear of collision with cars, but sidewalk riding is more dangerous in other aspects, due to uneven pavement and obstacles.
5. Many riders choose to ride too fast in order to keep pace with automobiles in order to avoid harassment.
6. Many of riders ride light racing bicycles in strained positions in order to attain higher speed (to avoid harassment), but those bicycles are harder to control.
7. Single-phase left turns are dangerous since the person needs to extend their left hand to signal, thus reduce control of their bicycle. It is doubly dangerous if they also have to cross tracks at the same time. Implementation of a two-phase left turn would reduce this hazard.
8. Motorists don't realize the consequences of their actions lead hazards for bicyclists. A swerve in front of a bicycle even without the bicycle and car colliding can result in the bicyclist losing control.
9. Traffic lights timed for cars mean that bicyclists race to get through them or risk stopping at every corner.
10. San Francisco is a hilly city and hills make bicycles more difficult to control. That's why you shouldn't try to direct bicyclists to ride on hills, which you've personally been doing when opposing the Fell/Oak project. Lack of infrastructure on the flat streets increases the volume of bicyclists trying for the hills.
11. Lack of infrastructure leads to fewer people bicycling, which leads to the sense of being alone. When there are no bicyclists around cyclists tend to make more unsafe maneuvers because there is no example and no peer pressure. Just like cars drive more slowly in a nice neighborhood with trees, bicycles ride more slowly when there is a relaxed vibe.
12. Conversely, narrow bicycle lanes cause those who need to pass to go too fast, and into traffic, which then brings up some other issues already mentioned.

The real question is how many bicycle solo falls are on a separated cycle track, like the panhandle. No report has looked specifically at solo injuries in cycletracks, but reports have shown an across the board reduction in injuries. Thus, you cannot make the conclusion you make.

At 2:37 PM, Anonymous Anonymous said...

This article explains why we need mandatory bike helmet laws more than we need separated bike lanes if you really want to improve the safety of bike riders.

At 12:38 PM, Blogger Rob Anderson said...

I respond to Mario's argument here.


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