11 February 2020

The Causes Of Deadly Car Accidents

Growing up, I spent a lot of time on country roads like these visiting family in rural Ohio and Michigan, and in the outskirts of the small town where I grew up. It doesn't surprise me that these almost unchanged road (which are also far from Level I trauma centers), give rise to a large share of motor vehicle accidents.
"Perhaps the biggest reason why we don’t see more fatal crashes on freeways is that there are no intersections on them (with a few exceptions). In fact, there are more drivers killed in intersections (20%) than on freeways."  
"After accounting for freeways (18%) and intersections and junctions (20%), we’re still left with more than 60% of drivers killed in automotive accidents left accounted for." 
"It turns out that drivers killed on rural roads with 2 lanes (i.e., one lane in each direction divided by a double yellow line) accounts for a staggering 38% of total mortality. This number would actually be higher, except to keep the three categories we have mutually exclusive, we backed out any intersection-related driver deaths on these roads and any killed on 2-lane rural roads that were classified as “freeway.” So, to recap, 3 of out every 4 deaths in a car occur on the freeway, at an intersection/junction, or on a rural road with a single lane in each direction."  
"In drivers killed on 2-lane rural roads, 50% involved a driver not wearing a seat belt. Close to 40% have alcohol in their system and nearly 90% of these drivers were over the legal limit of 0.08 g/dL. About one-third involved speeding, and 16% did not have a valid driver’s license."

Presumably, almost 24% of fatal accidents are on urban roads that aren't freeways, and smaller still rural roads, combined, outside of intersections.

The analysis at the linked article could be better. 

First, it doesn't factor in the volume of traffic on each kind of road. Fatalities per vehicle-mile and passenger-mile traveled is what we care about, not raw fatalities. There are fewer vehicle-miles travelled on rural two lane roads than there are on urban roads or freeways in the United States. 

As of 2017, about 34% of vehicle miles traveled were on freeways, 21% were on any other kind of public road in a rural area (of which, not more than 17% of the total were the kind of rural two lane roads being discussed), and 45% were on any other kind of public road in an urban area. So, the relative risk on rural roads may be a lot higher than the raw numbers provided suggest. Rural two lane road are on the order of four times more dangerous than urban roads that aren't freeways, and are also on the order of four times more dangerous than freeways.

Second, the article doesn't compare risk factors in accident cases with the frequency of those risk factors in non-accident cases

Speeding is very common in trips that don't give rise to accidents, so this risk factor is probably greatly exaggerated in the article. Speeding is "pervasive" and engaged in by most drivers in trips that don't give rise to fatal accidents. This is barely a factor of its own at all relative to the baseline rate.

High levels of intoxication are not common in trips that don't result in accidents and this certainly is a huge risk factor. "About 8 percent of drivers during weekend nighttime hours were found to have some alcohol in their system, and 1.5 percent were found with .08 percent or higher breath alcohol content[.]" The percentages are lower at nights during weekdays that aren't holidays and during the day. This compares to 36% of accidents involving drivers with more than 0.08 percent breath alcohol content who are in accidents on rural two lane roads (much more than 24 times the baseline rate).

About 11.4% of drivers don't wear seatbelts, while 50% of those who die in accidents don't (almost five times the baseline rate). So, clearly this is a significant risk factor for obvious reasons.

I don't know what proportion of trips that don't give rise to accidents are driven by non-licensed drivers, but it isn't 0%. A somewhat sketchy source claims that 10% of people on the road at any time don't have a license, which, if true, suggests that the relative risk of an unlicensed driver, while real, isn't as great as you might expect looking only at the raw numbers, in which 16% of fatal accidents on rural two lane highways involve unlicensed driers (1.6 times the baseline rate).

Finally, the article focuses exclusively on what you can do to protect yourself in the status quo, but the answer is that there are huge limits to what you can do to protect yourself from an accident where someone else is at fault, as is usually the case in accidents at intersections.

In those cases, civil engineers and urban planners play a huge role, and the main step that can be taken to reduce accidents is to replace stop sign and stoplight controlled small intersections with roundabouts, in which the kinds of accidents that account for 90% of fatalities in intersection accidents basically don't happen. Unsurprisingly, modern roundabouts in appropriate intersections reduce fatal accidents intersections by 90%.

No comments: