Category Archives: exposure

Pedestrians and cyclists can get hurt even on NYC sidewalks, but the heavier ones less so

The NY Times reports on an article re: pedestrian and cyclist safety from the Journal of Trauma and Acute Care Surgery.

-Gathering data from 1,400 people who have been injured is impressive.

-That pedestrians are most vulnerable in crosswalks is not surprising; it is where the pedestrians are–the whole “exposure” aspect.

-But now, we finally have a partial silver lining to the obesity epidemic in the US: excessive weight may prove a boon for pedestrians in a collision. Victims with an above-normal body mass index were found to have less severe injuries than their counterparts. “It is not implausible that a greater proportion of torso and extremity fat may protect against injury”

Health tradeoffs (some of them) hitting popular press

The Atlantic Cities has a diddy exposing some of the pollution ill-effects of cycling. But, the larger question is still left open. Even considering the air pollution burden from cycling–and perhaps even the safety risks owing to crashes–is it healthy? We need to look at the larger context. The prevailing evidence, I would argue, suggests that cycling is healthy–overall–because of the physical activity benefits.

Health benefits of switching to transport and bikes

Some of the most robust research, internationally, of the health benefits derived from switching car use to other modes is coming out of the Centre for Research in Environmental Epidemiology (CREAL) in Barcelona. They have looked at the impacts of Barcelona’s bike-sharing system in the past. Their latest work is more generally about the benefits of public transport and bike. Yes, they are working with future scenarios. Yes, there are lots of assumptions embedded. But the framework and the identification of key outcomes and specific measures is good to see.

Cover imageReplacing car trips by increasing bike and public transport in the
greater Barcelona metropolitan area: A health impact assessment study
Environment International
Volume 49, 15 November 2012, Pages 100–109
Rojas et al.


Estimate the health risks and benefits of mode shifts from car to cycling and public transport in the metropolitan area of Barcelona,Spain.
We conducted a health impact assessment (HIA), creating 8 different scenarios on the replacement of short and long car trips, by public transport or/and bike. The primary outcome measure was all-cause
mortality and change in life expectancy related to two different assessments: A) the exposure of travellers to physical activity, air pollution to particulate matter < 2.5 μm (PM2.5), and road traffic fatality; and B) the exposure of general population to PM2.5, modelling by Barcelona Air-Dispersion Model. The secondary outcome was a change in emissions of carbon dioxide.
The annual health impact of a shift of 40% of the car trips, starting and ending in Barcelona City, to cycling (n = 141,690) would be for the travellers who shift modes 1.15 additional deaths from air pollution, 0.17 additional deaths from road traffic fatality and 67.46 deaths avoided from physical activity resulting in a total of 66.12 deaths avoided. Fewer deaths would be avoided annually if half of the replaced trips were shifted to public transport (43.76 deaths). The annual health impact in the Barcelona City general population (n = 1,630,494) of the 40% reduction in car trips would be 10.03 deaths avoided due to the reduction of 0.64% in exposure to PM2.5. The deaths (including travellers and general population) avoided in Barcelona City therefore would be 76.15 annually. Further health benefits would
be obtained with a shift of 40% of the car trips from the Greater Barcelona Metropolitan which either start or end in Barcelona City to public transport (40.15 deaths avoided) or public transport and
cycling (98.50 deaths avoided).The carbon dioxide reduction for shifting from car to other modes of transport (bike and public transport) in Barcelona metropolitan area was estimated to be 203,251
t/CO2 emissions per year.

Interventions to reduce car use and increase cycling and the use of public transport in metropolitan areas, like Barcelona, can produce health benefits for travellers and for the general population of the
city. Also these interventions help to reduce green house gas emissions.

  • We assess the health impacts of replacing car trips by bicycle or public transport.
  • Replacement of the car trips reduces mortality in travellers who shift the mode.
  • Replacement of the car trips also reduces mortality in residents of urban areas.
  • Replacement of car trips can reduce the emissions of CO2.


The helmetless debate ensues, this time in the NYTimes

Where should we come down on this matter? Here is what I want to know:

(1) Where helmetless behavior reigns strong, what is the average speed of the cyclist?

(2) Where helmetless behavior reigns strong, what is the average speed of the auto?

Nothing ever talks about these matters. I imagine both are substantially slower than in most US settings. Here’s a proposition: bring down the speeds of both and helmetless behavior might not be such a big deal.

Guest post: Adjusting for variation in bike counts (contribution from Krista Nordback)

The following is a guest post from Krista Nordback, PhD Candidate in Civil Engineering at the University of Colorado Denver and member of the Active Communities / Transportation (ACT) Research Group——-

PictureWith the bicycle arms race underway (which is a good thing because peer pressure always helps communities do more), it’s really hard to know who is winning.  If you read the blurbs, every city claims to be winning because every city is seeing gains in their bicycle counts. But how consistent are the counting approaches? How robust are the counting approaches? Even with consistent and robust approaches, how does one account for geographic or climate variations. Does a high bike count in Minneapolis during a sunny and 70 degree day ensure the same in mid January?  Probably not.

What is the best way to  compare cities with high counts in the summer, and low counts in the winter to cities with balmy weather all year round?  One way, borrowed from the motorized traffic world, is to calculate an average daily count for the whole year (aka AADT).  The National Bike and Pedestrian Documentation Project has done just that, offering factors to annualize your hourly bike and pedestrian counts.  While this was a notable step forward 4 years ago, it’s far from definitive. 

First, the idea that we can create one set of factors for the entire country leads to major inaccuracies.  Clearly, cultural, climate and terrain vary from city to city, which impact riding habits.  Furthermore, it may lull cities into thinking they don’t need their own continuous automated counts at all since it’s being done at the national level.

Second, annualizing counts based on a one or two hour count inherently lead to more inaccuracies.  There’s a reason traffic engineers abandoned the practice decades ago.  Even with relatively stable traffic counts, one or two hour counts leads to wildly varying estimates.  Basing estimates of annual average daily bicyclists (AADB) on one hour counts can be off by as much as six times actual AADB! 

Here’s the good news!  Cities around the country are installing their own automated bicycle and pedestrian counters that capture traffic 365-24-7.  Permanent automated counts sites provide cities the data they need to create their own, city specific annualization factors.  And, portable automated counters can count for a week at a time at various locations around the city giving a much better estimate of volumes at the location than an army of well meaning volunteers.

The time has come for the bicycle community to realize what motor traffic engineers have known for decades.  Too small a sample size (e.g., a 2 hour bike count) can be WORSE than nothing.  Let’s put those well-meaning volunteers to work doing something more meaningful, like moving, protecting, and maintaining our automated bike counters.  Only then can we robustly compare bike counts on the Midtown Greenway in Minneapolis to those on the Lance Armstrong Bikeway in Austin.

CDC Expert Panel on active transportation

The Centers for Disease Control and Prevention is presently convening an Active Transportation Expert Panel for a 2-day meeting. I helped serve on the planning committee for the workshop and offered one of the presentations titled, “Measuring Active Travel: Perspectives from the Transport Field,” with some key slides below.

Advancing “total health:” shining light on somewhat competing issues of physical activity & air pollution exposure

What if a community had all the successful ingredients leading to higher rates of cycling and walking (e.g., population density, intersection density, diverse mix of land uses, bike paths galore, etc)? Would planners then being doing their job? How would this relate to the total health for residents?
A small but growing number of studies are looking at two criteria of health simultaneously: exercise and air pollution. The results suggest these things might not always move together in the same direction—a “wake up” call for planners who have typically been obsessed with increasing physical activity. This study helps bring to light that the health benefits from increased physical activity in highly walkable neighborhoods may be offset by adverse effects of air pollution exposure. In the words of one of the co-authors, “city planning efforts have been planning to optimize one risk factor [lack of physical activity], when there are multiple risk factors to be taken into account.” <just fyi, another health consideration is bicycle/traffic safety, but that issue might be less controversial>
Should we worry about this? Of course. Is it a growing issue that has the potential to further divide planning initiatives? Hopefully not. Two possibilities:
·         Will cleaner cars, cleaner businesses, and cleaner everything else coming on-line possibly lessen the need to be concerned about pollution.
·         Is the fact that the study is based in Los Angeles—a basin that has perennially been out of compliance with EPA standards and probably has a disproportionate share of polluting car use (both in terms of sheer use and % of fleet that is old)—reason to suggest the issues there are not as bad as other places?
It is hard to say. I don’t think the solution is pollution filter face masks. This work merely suggests an area worth of further investigation to ensure we are not shooting ourselves in the foot.
Background: Physical inactivity and exposure to air pollution are important risk factors for death and disease globally. The built environment may influence exposures to these risk factors in different ways and thus differentially affect the health of urban populations.
Objective: We investigated the built environment’s association with air pollution and physical inactivity, and estimated attributable health risks.
Methods: We used a regional travel survey to estimate within-urban variability in physical inactivity and home-based air pollution exposure [particulate matter with aerodynamic diameter ≤ 2.5 μm (PM2.5), nitrogen oxides (NOx), and ozone (O3)] for 30,007 individuals in southern California. We then estimated the resulting risk for ischemic heart disease (IHD) using literature-derived dose–response values. Using a cross-sectional approach, we compared estimated IHD mortality risks among neighborhoods based on “walkability” scores.
Results: The proportion of physically active individuals was higher in high- versus low-walkability neighborhoods (24.9% vs. 12.5%); however, only a small proportion of the population was physically active, and between-neighborhood variability in estimated IHD mortality attributable to physical inactivity was modest (7 fewer IHD deaths/100,000/year in high- vs. low-walkability neighborhoods). Between-neighborhood differences in estimated IHD mortality from air pollution were comparable in magnitude (9 more IHD deaths/100,000/year for PM2.5 and 3 fewer IHD deaths for O3 in high- vs. low-walkability neighborhoods), suggesting that population health benefits from increased physical activity in high-walkability neighborhoods may be offset by adverse effects of air pollution exposure.
Policy implications: Currently, planning efforts mainly focus on increasing physical activity through neighborhood design. Our results suggest that differences in population health impacts among neighborhoods are similar in magnitude for air pollution and physical activity. Thus, physical activity and exposure to air pollution are critical aspects of planning for cleaner, health-promoting cities.