Unwelcome news - HEVs do hit more pedestrians & cyclists

[samiam]

I really didn't want to read this bad news, but facts are facts boys and girls. We are apparently hitting more pedestrians and cyclists with our Prii than drivers of ICE vehicles. These are the latest data from The National Highway Traffic Safety Administration.

You can read the whole report free at this link:
http://www-nrd.nhtsa.dot.gov/Pubs/811204.PDF

But here are the highlights:
"This study found that pedestrian and bicyclist crashes involving both HEVs and ICE vehicles commonly occurred on roadways, in zones with low speed limits, during daytime and in clear weather, with higher incidence rates for HEVs when compared to ICE vehicles.

A variety of crash factors were examined to determine the relative incidence rates of HEVs versus ICE vehicles in a range of crash scenarios. For one group of scenarios, those in which a vehicle is slowing or stopping, backing up, or entering or leaving a parking space, a statistically significant effect was found due to engine type. The HEV was two times more likely to be involved in a pedestrian crash in these situations than was an ICE vehicle.
(emphasis added)

Vehicle maneuvers such as slowing or stopping, backing up, or entering or leaving a parking space were grouped in one category assuming these maneuvers have occurred at very low speeds where the difference between the sound levels produced by the hybrids versus ICE vehicles is the greatest. In future analysis with a larger sample size, it would be ideal to investigate each of these maneuvers individually.

Incidence rate of pedestrian crashes in scenarios when vehicles make turns was significantly higher for HEVs when compared to ICE vehicles. There was no statistically significant difference in incidence rate of pedestrian crashes involving HEVs when compared to ICE vehicles when both type of vehicles were going straight.

Similar to pedestrians, in crashes that occurred at very low speed, such as when a vehicle is making a turn, slowing or stopping, backing up, or entering or leaving a parking space, the incidence rate of bicyclist crashes involving HEVs was significantly higher when compared to ICE vehicles. On a roadway was the most common location of bicyclist crashes involving both HEVs and ICE vehicles with no statistically significant difference. On the other hand, bicyclist crashes involving HEVs at intersections or interchanges were significantly higher when compared to ICE vehicles."

Bummer!

[bwilson4web]

Relax,

This report suffers a seriously flawed methodology:

Originally Posted by pp_8

Incidence rates were calculated as the number of vehicles of a given type involved in crashes with pedestrians or bicyclists under certain scenarios, divided by the total number of that type of vehicle that were in any crashes under the same scenarios. . . .

What they calculated was the distribution of types of accidents for different classes of vehicles, not a rate proportional to the actual number of vehicles in the targeted states. What this means is we can not tell the relative risk of HEV to non-HEVs because they did not use the population of all vehicles.

Let's start with their numbers:

HEV accidents	ICE vehicle accidents	total	HEV %	ICE %
8,387	559,703	568,090	1.4%	98.5%

.
Now let's add the absolute numbers of HEVs versus ICE vehicles:

HEV vehicles	ICE vehicles	total	HEV %	ICE %
??	??	??	??	??

.
Wait a minute ... what is the ratio of HEVs, actually their miles traveled to ICE vehicles? ... It is missing!

Without the population of hybrids and ICE vehicles, no one knows if perhaps ... just perhaps ... the HEVs are having half the number of recorded accidents as the ICE vehicles. That data is not in this report, is it?

If you have a small population compared to a large population, this is comparing the ratio of nose and ear sizes within each population without determining how many of each group are causing the problem, accidents. The ratio of types of accidents means the distributions may be different but if the worst relative case, is half the rate of the other vehicles, it doesn't matter. Until we have the rates relative to the total numbers of HEV and ICE vehicles, this tells us nothing about the relative risks of each type of vehicle.

The other obvious question is the distribution of these incidents by vehicle body-style. The reason this is important is some body styles have less visibility than others and are more likely to obscure pedestrians and bicyclists. The "backover" special investigation report was specific in pointing out the problem of poor visibility, especially to the rear, of some car styles ... having nothing to do with noise.

Then there is the problem of small numbers in pedestrian accidents (table 6a):

Vehicle Maneuver	Pedestrian count - HEVs	Incidence rate of pedestrian crashes - HEVs	Pedestrian count - ICE vehicles	Incidence rate of pedestrian crashes - ICE vehicles
Going straight	33	0.9%	2,069	0.8%
Making a turn (*)	19	1.8%	698	1.0%
Slowing/stopping	6	0.5%	148	0.2%
Backing	7	5.3%	261	2.9%
Entering/leaving parking space/driveway	1	1.2%	55	0.9%
Starting in traffic	3	2.9%	50	1.2%
Other	6	0.3%	238	0.2%

.
Does anyone notice the number of turning HEV count is larger than the number of backing incidents but the percentages are reversed? How do "19" Pedestrian incidents become a larger percentage than the "7" backing incidents. I have no idea how they came up with these numbers. Equally bad, these are laughably small numbers to draw any conclusions ... especially "backing up". Sorry but my eyes see a really bad problem here.

Just for grins, I did a Bob Wilson style of table 6a:

Vehicle Maneuver	Pedestrian count - HEVs	Incidence rate of pedestrian crashes - HEVs	Pedestrian count - ICE vehicles	Incidence rate of pedestrian crashes - ICE vehicles
Going straight	33	44.0%	2069	58.8%
Making a turn (*)	19	25.3%	698	19.8%
Slowing/stopping	6	8.0%	148	4.2%
Backing	7	9.3%	261	7.4%
Entering/leaving parking space/driveway	1	1.3%	55	1.6%
Starting in traffic	3	4.0%	50	1.4%
Other	6	8.0%	238	6.8%

.

Before sack cloth and ashes, read the report and see the real numbers, the absolute numbers, and realize they have not counted the rates of accidents versus the population of vehicles.

Fortunately, we can file a Freedom of Information Act request and ask for the missing data. ... And share our concerns that this is a half-baked report missing critical data. ... More to follow after I get a chance to read it in more detail later this evening. <grins>

There is a wide spread problem of too many folks not understanding math and numbers
because it means they can be baffled by numerical nonsense. The truth is in the tables and just a cursory glance shows this report to be at best, incomplete, and possibly, misleading.

Bob Wilson

[chogan2]

We've been waiting to for this, but they did some odd things.

First, they excluded the Insight from the study because it can't move without running the ICE. But they included the HCH. I thought the HCH couldn't move without running the ICE.

Second, they list the Toyota Corolla as part of the HEV group. There was never a Corolla hybrid, was there?

Third, they didn't actually calculate the crash rate (crashes per vehicle mile or crashes per vehicle). What they actually calculated was the fraction of crashes the involved pedestrians and bikes. That's just ... odd. I can't quite get my mind around that except that it's expedient.

Fourth, they controlled for weather and lighting, but not urbanicity. My understanding is that low-speed pedestrian crashes are far more likely in urban areas. But looking at their GESS database, it looks like information on urbanicity is missing for better than a third of the cases. So it's possible they couldn't correct for that (though you think they'd have the ZIP of the accident location in most cases).

The "statistical significance" of the results appears to be controlled almost entirely by the number of cases. When the split the data along various dimensions, the small cells are unlikely to be "statistically significant". The only significant exception is for impacts while turning, which appears for pedestrians but not for bikes.

In particular, I don't think you can make anything of the under-35-mph/over-35-mph split, as there were just too few accidents in the over-35 category to give any results at all.

That said, they found the effect where they expected to find it. If I were handed the task, I'm not sure I'd have done anything that differently, other than for the possible technical error with the HCH.

The passage summarizing the cars involved is here:

"The HEVs (case group) selected were the Toyota Corolla, Toyota Camry, Toyota Prius, Honda Civic, and Honda Accord. The ICE vehicles (control group)
selected are the Toyota Corolla, Toyota Camry, Honda Civic, and Honda Accord. The analysis is limited to vehicles of model year 2000 and later. The Honda Insight was considered but excluded from the analysis due to the fact that the earlier model years of Honda Insight always operate using their internal combustion engines, even at low speed."

Actually, now that I read that, it's not even weighted. So on a weighted-average basis, what you are seeing is, I think, essentially a comparison between Prius (most popular HEV) and whatever the most popular ICE car on that list is.

I think urbanicity is important, and if I can find the data on urbanicity and pedestrian accidents I'll post it here.

EDIT: Bob, I don't think your point about the numbers is correct, although it's admittedly difficult to get your hands around the numbers. Those rates are the ratio of pedestrian accidents to all accidents of that type. The denominator is different for every cell in the table. What they are saying on the line that looks odd to you is that, of all incidents involving backing up, for the HEVs, 5.3% involved a pedestrian.

So, yes, the numbers are small. I'm sure they normed this to all accidents rather than all cars as a matter of expediency. (Not sure I'd have done it differently, given the work involved in estimating the car fleet by state). The apparent technical errors in the car lists gives me pause -- but they could be a typo, for all I know. The "matching" gives me pause -- looks like they didn't actually have a matched sample of HEV and non-HEV, looks like they had two pools of cars, and whatever showed up in this pools is what they analyzed. So the the HEV list is (by N of observations) heavily weighted to the Prius, while the non-HEV list would have to be mostly Camrys, I'd guess. Not clear that makes any real difference.

And, yeah, your point about these rates not being per car or per vehicle mile is well taken. In their, they results might be from Prii having relatively few total accidents in a class (small denominator) instead of relatively many pedestrian accidents (large numerator). Yet, that seems fairly implausible to me. That the stars would align to create these results.

OK, one more esoteric statistical issue, in that this is just 12 states. Really, this is a clustered sample. The hybrid list has to be dominated by the Prius, and Prius ownership rates (particularly at that time) were highly geographically concentrated. To the list of things that might be true, the results might be an artifact of geographic location. But to tell you the truth, aside from fatalities, I'm not sure there's any comprehensive data on pedestrian accident rates by state (not even sure if you could look at that).

But the bottom line is that they saw the effect right where they expected to see it. Yeah, it's not the strongest result, but ... having done my share of studies with thin data, I wouldn't dismiss it out of hand. If this is all the information available at the time the Congress has to make the decision, then the slender weight of the evidence suggests that it is an issue.

In terms of what's feasible, for the ICE cars at least, I'd at least like to have seen the spread of pedestrian-as-%-of-total across the makes of cars. I mean, basically, given the time period, this is essentially a comparison of Prius versus other cars (weighted average basis). What's the likelihood that you'd see some statistically significant results (given the fairly large N of tests performed), if you just took two of the ICE cars in the ICE pool and did the same comparison?

I'll keep thinking about it and share any further insights. I mean, it's not implausible. The numbers are suggestive. The report makes it clear that this is not the last word. If the Congress acts soon, this is moot, but if there is going to be another round, with possibly another study, it might do some good to offer constructive criticism. But given the small numbers, there's only so much you can do with this.

FURTHER EDIT: I was just quietly corrected on the HCH thing -- they can move at low speeds without the engine firing, which therefore puts them in the same low-noise class as the Prius at low speeds.

[SageBrush]

BW's arguments likely do not hold up. The 'relative' in "relative incidence" looks like a normalization of vehicle type on the road.

A couple of comments occur to me:
1. What is the relative risk of HEV's to SUV's ? Or trucks ? How about cars where the driver sits so low in the cabin that only the driver's forehead reaches the window level ? If the public is interested in vehicle traits that endanger pedestrians, lets talk bumpers and grills. Or ICE/vehicle weight. In short, they should be setting up a multiple regression analysis, not this hack job.
2. Prius does not have a valid ICE only comparison car. How do the numbers look without it ? or compared to split-wing hatchbacks ?
3. Any attempt to normalize driver demographics ?

.. Unrelated to the weaknesses of this study -- I have to admit I always thought that reverse beeping *inside* the cabin was a bit silly.

[bwilson4web]

Originally Posted by chogan2

. . .

EDIT: Bob, I don't think your point about the numbers is correct, although it's admittedly difficult to get your hands around the numbers. Those rates are the ratio of pedestrian accidents to all accidents of that type. The denominator is different for every cell in the table. What they are saying on the line that looks odd to you is that, of all incidents involving backing up, for the HEVs, 5.3% involved a pedestrian. . . .

I tried that approach and it really goes 'off the scale:'

Vehicle Maneuver	Pedestrian count - HEVs	Incidence rate of pedestrian crashes - HEVs	Pedestrian count - ICE vehicles	Incidence rate of pedestrian crashes - ICE vehicles	total within group
Going straight	33	1.57%	2069	98.43%	2102
Making a turn (*)	19	2.65%	698	97.35%	717
Slowing/stopping	6	3.90%	148	96.10%	154
Backing	7	2.61%	261	97.39%	268
Entering/leaving parking space/driveway	1	1.79%	55	98.21%	56
Starting in traffic	3	5.66%	50	94.34%	53
Other	6	2.46%	238	97.54%	244

.
When I sum the number of accidents for HEV and ICE, we get a nice large number for the denominator but the calculated percentages, incidents/total count of incidents, really throws the percentages off. It becomes even more impossible.

I'm open to any suggestions on how the 'percentages' in table 6a were generated. I have no problem with the raw data but the calculated percentages in table 6a to my eyes are catty-wampus. I still don't see where the denominator(s) come from. For example,

2,069 - Pedestrian count - ICE vehicles going straight
0.8% - Incident rate
258,625 - the denominator to give 0.8%

132 - Pedestrian count - ICE backing
2.9% - Incident rate
9,000 - the denominator to give 2.9%

I just don't see where these denominators are coming from.

Bob Wilson

[bwilson4web]

Originally Posted by SageBrush

BW's arguments likely do not hold up. The 'relative' in "relative incidence" looks like a normalization of vehicle type on the road.
. . .

I'm good with that if there were a table in the report showing these numbers but all I can find is this quote standing alone:

Nationwide registrations for new HEVs rose to 350,289 in 2007, a 38-percent increase from 2006, according to R.L.Polk & Co.

This is only HEVs and does not include ICE registrations and it is only a rate of increase, not an absolute number of vehicles registered. So it tells us nothing about "normalization of vehicle type on the road."

If you can explain the calculations in table 6a, please do. I don't see where those percentages came from and those are the ones used for the claims of higher turning and backing rates.

Bob Wilson

ps. Please don't take my questions as anything but curiosity. I'm willing to learn but right now, the numbers in table 6a, the percentages, don't make a lick of sense.

[Celtic Blue]

Originally Posted by SageBrush

Unrelated to the weaknesses of this study -- I have to admit I always thought that reverse beeping *inside* the cabin was a bit silly.

It's worse than silly, it is distracting/irritating to the driver and as a result likely to cause more incidents than it prevents. I disabled it for that reason. Might as well have a telephone ringing inside the car while you back, it is that stupid of a distraction. Toyota needs to figure out who championed this feature and give them a permanent vacation.

[SageBrush]

If you can explain the calculations in table 6a, please do

No problem, happy to help you get a clue: They are using ~ 70:1 ICE:HEV on the road. Rounding errors from only reporting one place after the decimal makes the arithmetic messy.

What's the likelihood that you'd see some statistically significant results (given the fairly large N of tests performed), if you just took two of the ICE cars in the ICE pool and did the same comparison?

Very valid [sic] point. Similar statistical miscues are rife in the medical literature, like HLA disease associations. Check 20, get one positive association for free

[hyo silver]

Nope, not what we wanted to hear. Forgive me for not being an astute statistician, but I'm not sure how to interpret the conflicting reports. Sam says one thing and Bob says another, and I don't want to have to choose sides.

[a_gray_prius]

I'd be interesting to see what the results look like distributed according to driver age, given that hybrids tend to be driven by older drivers.