What is left out of this study?: "UK cities should have more 20mph speed zones, as they have cut road injuries by over 40% in London"

Here is the claim:

the number of children killed or seriously injured has been halved over the past 15 years, the British Medical Journal reported.

The London School of Hygiene and Tropical Medicine study estimates 20mph zones have the potential to prevent up to 700 casualties in London alone.

At 20mph, it is estimated only one in 40 pedestrians is killed in a crash.

This compares with a one in five chance for someone hit at 30mph.

The researchers compared data on road collisions, injuries and deaths in London between 1986 and 2006, with speed limits on roads.

After adjusting for a general reduction in road injuries in recent years, they found that the introduction of 20mph zones were associated with a 41.9% drop in casualties. . . .

Note the claim that per 40 people killed, the number dead will fall from eight to one when cars go from traveling at 30 mph to 20 mph. That is an 87.5 percent reduction, compared to the 41.9 percent drop they claim occurred. Taking into account that accidents generally tend to fall over time is helpful, but it seems likely that people are more likely to be careful where the risks are greater. The faster the cars are speeding in an area, the less likely that parents will leave their kids walking around there.

It would be interesting to see some estimate of the cost of reducing speeds by one third. What is the increase in hours that people will spend in cars? Obviously the increase in the short run is likely to be greater than the increase in the longer run. Apparently there were about 2.7 million cars in London in 1992 (in a fast search, I couldn't find a more recent number). Cutting the speed limit from 30 to 20 mph means that it takes 50 percent longer to make a trip. Take some completely made up numbers, simply to give an idea of the magnitudes involved. If each car on average spends two hours per week on these particular streets, that means 52 additional travel hours per year. Even if cars only have 1 person in the car at a time and their time costs on average are just $20 per hour, that comes to about $2.8 billion in lost time costs. Now assume that there are 700 lives that would be saved and that the average value of a life is $5 million, that comes to a total saving of $3.5 billion. Both of these numbers are quite close. If on average 1.5 people are traveling in a car, the $2.8 billion becomes $4.2 billion. I don't know what the final conclusion is here, but the answer is hardly the "should" claimed by this study, even assuming that the study was done properly and I can't make that judgment (though it is a medical journal and I would guess that they didn't get it right).