The county has made an emergency order closing Waipi‘o Valley Road to everyone except residents and farmers. It is based on an engineering consultant’s estimate of the risk of rockfalls.
That estimate is wrong. It is about 280 times too high for pedestrians and 100 times too high for motorists.
I don’t expect a reader to believe me rather than a team of professional engineering consultants without an explanation. It takes only common sense and a little bit of math to understand the real risk.
The consultant estimated that in 18,000 pedestrian trips, on average one person will be killed by a rockfall, and one person would be killed in 170,000 vehicular trips. The consultant assumed that every pedestrian hit by a rock would be killed, and three in 10 vehicular impacts would kill someone.
Pre-COVID, we had about 50,000 pedestrian trips per year on the Waipi‘o road. With the one in 18,000 estimate, you would expect about 2.8 pedestrians per year to be killed by a rockfall. That’s 14 deaths in five years. No one I talk to has heard of any.
If you apply statistics, the chance is close to zero that you would go more than two years without a rockfall fatality if the consultant’s estimate is right. The consultant estimates six rockfalls per year. They don’t mean any rockfall, just ones big enough to kill someone. If they are right, 2.8 of those six rockfalls — almost half — would have to hit someone.
Take a mental snapshot of the Waipi‘o Valley Road. At any time, pedestrians and cars take up only a tiny fraction of the road. The vast majority of the time, a falling rock will hit nothing but road. There is no way that almost half the rocks are going to hit someone.
What is the real risk? Think about a rock tumbling toward the road. An approaching vehicle can be hit only during the time it takes to cross the rock’s path. Any other time, the rock misses.
Let’s use the consultant’s numbers. The average vehicle is about 15 feet long and travels at 15 feet per second on the Waipi‘o road, or about 10 mph. The vehicle can be hit during the one second it takes to cross the rock’s path.
Pre-COVID, about 174 vehicles used the road on an average day. If a rockfall happened that day, the vehicles were at risk for a total of 174 seconds. There are 86,400 seconds in a day. If you divide 174 by 86,400, you get about 1/500. That is the chance that any single rockfall will hit a vehicle. It’s very unlikely.
The consultant estimated six rockfalls per year, so that means there is a 6/500 chance in any one year that a vehicle will be hit. Multiply that by the consultant’s numbers of 65,310 vehicles per year. The result: one in 5 million vehicles will be hit. Using the consultant’s estimate that three in 10 vehicular impacts would cause a fatality, the real risk of death to a vehicle occupant is about one in 16 million trips. That’s about a hundred times less than the consultant’s estimate.
You can do the same calculation for pedestrians, using the correct velocities, widths, and numbers per day. The result: one in five million pedestrians will be hit. The consultant’s estimate of 1/18,000 is about 280 times too high.
The consultant misapplied a formula. I can explain how — I’ve emailed them and the county explaining it — but it’s too technical to do it here.
The county should not rely on an erroneous study. Their entire risk assessment depends on these faulty numbers.
If you properly calculate the real risk, is it acceptable? Everything involves some risk. For example, you have a risk of being killed in an accident driving from Hilo to the Waipi‘o Valley lookout. Using the average risk of death from driving, it’s about one in 1.8 million. It’s higher than the rockfall risk for a pedestrian or driver going down the Waipi‘o Valley Road.
The risk of rockfall is certainly high enough to justify remedial work on the slopes. It is not high enough to justify banning the general public for years from Waipi‘o, a place that we all cherish and should be able to enjoy.
There are hazards on the Waipi‘o road. It is very steep, narrow, and has severe drop-offs. These hazards — which have existed for a long time — do not create an emergency. They were not part of the consultant’s report, which is only about rockfall risk. They can be reduced with better traffic management, but don’t justify closing the road to the general public. And pedestrians are not the problem.
If the consultants or the county think that what I am saying here is wrong, they should explain how they got their numbers in the same detail that I do so members of the public can judge for themselves.
I’m writing this as a private citizen and not in any official capacity.
Chris Yuen is a member of the state Board of Land and Natural Resources.
