More reading, although I'm having difficulties getting links for pdfs:

"Motorcycle Conspicuity and the Effect of Fleet Daytime Running Lights"

> The annual number of motorcycle rider fatalities in the United States has more than doubled from 2294 in 1998 to 5290 in 2008 (National Highway Traffic Safety Administration, 2009). Many multi-vehicle motorcycle crashes involve right-of-way violations where another vehicle turns in front of, or crosses the path of an on-coming motorcycle. Improving the frontal conspicuity of motorcycles with forward lighting may reduce these types of crashes. On the other hand, widespread use of DRL on passenger vehicles may reduce the safety effectiveness of daytime headlamp use by motorcyclists. Research is needed to address these questions. This study involved testing the Fleet DRL Hypothesis that widespread use of daytime running lights (DRL) among the motor vehicle fleet is associated with an increased risk for certain types of multi-vehicle motorcycle crashes. This hypothesis is based on the assumption that the conspicuity of motorcycles (which normally run with their headlamp illuminated all the time) is effectively reduced during the daytime when a high proportion of other vehicles have DRL illuminated. To test the hypothesis, crash data from Canada where DRL use is mandatory were compared to crash data from the northern United States where DRL use is not mandatory and fleet penetration of DRL has been modest. Based on several specific assumptions, we developed a set of ten testable predictions that follow from the hypothesis. We compared crash data from the Fatality Analysis Reporting System (FARS) for 24 northern United States for the period of 2001 – 2007 to fatal crash data from the Canadian National Collision Data Base (NCDB) provided by Transport Canada for the same years. Crash scenarios that were plausibly relevant to frontal conspicuity of the involved vehicles were defined as DRL-relevant. The proportion of DRL-relevant crashes was modeled by country, year, and whether the crash involved a motorcycle. We fit separate models for crash data that occurred in four groups defined by time of day (Day, Night) and location (Rural, Urban) of the crash. The results supported seven of ten predictions indicating that the Fleet DRL Hypothesis may be true for urban roadways (but may not true for rural roadways). These results suggest that there could be negative consequences for motorcycle riders of widespread DRL use in the vehicle fleet. For urban roadways especially, the proportion of two-vehicle fatal motorcycle crashes that are relevant to frontal conspicuity of the vehicles (DRL relevant) is higher in Canada than in the USA. This result and other related predictions verified by the modeling results support the Fleet DRL Hypothesis for urban roadways, that widespread use of DRL in the vehicle fleet increases the relative risk for certain types of multi-vehicle motorcycle crashes. This conclusion should be interpreted cautiously in light of some limitations of the analysis approach.

"The Effects of Motor Vehicle Fleet
Daytime Running Lights (DRL) on
Motorcycle Conspicuity"

> As a group, motorcyclists experience a high fatality rate, a significant number of which are attributable to right-
of-way violations by other drivers. One factor behind the high crash rate is insufficient conspicuity of
motorcycles, which is now of greater concern because of the increasing use of Daytime Running Lights (DRL)
in the vehicle fleet. The hypothesis is that the additional lights on all vehicles will degrade the conspicuity of
the previously unique DRL signal used by motorcycles. The main goal of the current study was to evaluate the
effects of motorcycle conspicuity treatments on other drivers’ left turn gap acceptance. This study was
comprised of three phases. In Phase 1, a test track study measured participants’ left turn gap judgment as a
function of motorcycle DRL treatments. This study was designed to determine which treatments yielded the
largest gaps, thereby making that treatment a good candidate for the on-road portion. No treatment was clearly
better, so lighting systems currently in use on motorcycles were selected for the on-road study. In Phase 2, an
on-road study measured gap acceptance, then followed up with intercept surveys of observed drivers. This
phase included data collection in the United States (low fleet DRL use) and Canada (high fleet DRL use) in
order to evaluate the effect of DRL use in the vehicle fleet. However, due to concerns about the comparability
of the U.S. and Canadian data, the results are inconclusive, and additional research is suggested. In Phase 3,
motorcycle side conspicuity treatments (retro-reflectors and marker lamp) were compared. Results indicated
that there were no differences in detection distance between the treatments.

"Effects of 24-Hour Headlight Use on Traffic Safety"

> Effect of Automobile DRLs on Motorcycle Safety
Minnesota law requires motorcyclists to use their headlights during daylight hours, and opponents of DRLs have
argued that requiring headlight use for all vehicles could make motorcycles less conspicuous. A NHTSA study on
this topic, Motorcycle Conspicuity and the Effect of Fleet Daytime Running Lights, is expected to be complete by the
end of 2010.

> The findings of the 2008 NHTSA study regarding motorcycles were not statistically significant, and the 2004
NHTSA study found that DRLs reduced daytime opposite direction fatal crashes of a passenger vehicle with a
motorcycle by 23 percent.