Page 9 - CEGE Magazine - Fall 2021

Their recalibration formula is fairly simple in principle, but applying it to particular cases requires (1) a plausible mecha- nism that describes how a safety-related modification works, and (2) a database reflecting how various elements (for example, initial positions of vehicles, speeds, drivers’ reaction times, or braking rates) are distributed among
the various types of crashes. The Davis group is continuing to research and build a database of the details for various types of crashes, and to determine how those details correspond or differ within the two identified populations of drivers, human-only and human + AV.
Micro-simulation
Assessing the transferability of a crash modification factor to a new situation requires an explanation of how the modification achieves its effect. Davis developed a theoretical framework for posing and testing explanations of how a specific crash modification achieves
its effect and how that explanation might be tested using micro-simulation. He is applying this approach to explain how various crash modifications might impact the likelihood of crash occurrences.
Davis (2019) described the use of micro-simulation to develop an ex- planation of how pedestrian hybrid beacons (PHB) modify pedestrian
crash likelihood. Since the literature indicated that PHBs can affect both pedestrian and driver behavior it was necessary to include both possibilities
in the model. To simulate injury severity distributions similar to those recorded
in a crash database, it was necessary
to propose that almost all simulated drivers attempt to brake in pedestrian/ vehicle encounters. Then, changing the simulated fraction of careful pedestrians from 0–30% to 80–90% gave simulated crash modification factors similar to estimates reported in the literature. The resulting working hypothesis, then, is that PHBs achieve their crash reduc- tion effect in large part by modifying pedestrian behavior. This is not so much a direct observation as it is an inference to the best explanation. That is, the support for the hypothesis comes from its ability to explain the data at hand. Davis proposed additional tests that
University of Minnesota College of Science and Engineering | DEPARTMENT OF CIVIL, ENVIRONMENTAL, AND GEO- ENGINEERING 9
TRANSPORT OF IDEAS
The biggest issue facing transportation engineers right now is how to account for partially or fully automated vehicles and other new mobility modes. An issue for Davis, given his interest in models and tools to predict safety consequences of various highway engineering decisions, is if we do not yet understand how AVs will affect traffic, how can we make good system decisions?
This was the problem Davis has been wrestling with for a while. He gave this insight into the workings of a researcher’s mind: “Once you get a problem in mind, to some extent you look directly for solutions. Yet in another way, once you have committed to a problem, almost everything that comes by you gets filtered through that question. You are always on the lookout for any dribble of information that might be useful.”
What helped Davis in this instance was a timely seminar on mecha- nisms and evidence-based medicine put on by the History of Technol- ogy and the Philosophy of Science department at UMN. A philosophy professor talked about how medical researchers try to understand diseases. The speaker addressed the medical approaches of mecha- nisms (for example, understanding the mechanism by which a vaccine interacts with a virus) and evidence-based medicine (for example, assessing the effectiveness of a vaccine with a clinical trial). An ongoing debate involves the importance of mechanism-based understanding
to clinical practice, and a case can be made that a mechanism-based understanding can be helpful in deciding when the findings of a clinical trial can be generalized to populations not included in the trial.
“AS I LISTENED,” SAID DAVIS, “A LIGHTBULB WENT OFF, AND I THOUGHT, THAT IS EXACTLY WHAT I HAVE BEEN CASTING AROUND FOR IN THE LAST COUPLE YEARS!”
In medical research the bind seems to be, when can a conclusion from one clinical trial transfer successfully to other populations? At an abstract/structural level, Davis saw that his questions could be ap- proached with something like the medical research model: when can the statistical summary data of the last 20-30 years be applied to a new population of drivers?
“As I listened,” said Davis, “a lightbulb went off, and I thought, that is exactly what I have been casting around for in the last couple years! If we understand enough about the mechanisms of crashes, then we might be able to move that model from human drivers to AVs.”
“This led us to analytic work done by researchers at UCLA, on iden- tifying conditions justifying when causal knowledge learned in one situation could be applied to other, different situations, which then led to the paper Jingru Gao and I wrote on how data on the distributions of crash surrogates could, in principle, be used to recalibrate crash modification factors.”