SlightlyScruffy t1_ix7k9yk wrote
The scheme did slightly reduce vehicular traffic, and very slightly reduced speeds, which between them must have had an effect on air quality too.
Competitive_Ninja839 t1_ix84n8a wrote
Could the reduced vehicular activity just be drivers choosing other roads with higher speed limits?
_DeanRiding OP t1_ix88lvx wrote
Possibly. I know Google Maps definitely tries to avoid 20mph roads on my route home from work despite those roads having less traffic and actually being quicker.
Competitive_Ninja839 t1_ix89ao1 wrote
That's Google maps for you. Meanwhile where I live, it recommends one of the most dangerous highways in America over a safe state route because it shaves 3-4 minutes off an hour commute.
seamustheseagull t1_ix7xl9k wrote
Casualities too. The conclusion mentions no significant change in casualties, however the data suggests otherwise, especially for pedestrians.
The low sample size means it's far from definitive, but certainly enough of a differential to warrant consideration.
A reduction in traffic volumes + a potential/likely decrease in pedestrian casualities suggests that this is a good intervention to consider for city centres.
[deleted] t1_ix80nf2 wrote
The data does not suggest otherwise. As you pointed out the sample size is way too low to draw any reasonable conclusion. So i don't know why in your next sentence you completely ignore that and continue on as if it does.
If an intersection only has 3 incidents per year on average and then after the study there are 2 incidents, it doesn't mean we can conclude that there was a reduction because of the speed limit.
seamustheseagull t1_ix87oe4 wrote
No, but it may warrant further consideration. Certainly not conclusive enough to say, "no change was noted".
It's a common false-negative problem. "The differences were statistically insignificant" is only valid when you have a reasonable sample size. While technically it is correct to say, it implies a conclusion where none can be drawn.
[deleted] t1_ix92xnk wrote
>it implies a conclusion where none can be drawn.
No effect is literally a conclusion that can be drawn.
fatsynatsy t1_ixbw3n2 wrote
inappropriately drawn... as the sample size is too small to find a small effect with statiscal significance.
[deleted] t1_ixcpk6h wrote
Because the amount of accidents in the first place is small. So reducing speed limit isint going to have an effect since the effect was so small in the first place.
fatsynatsy t1_ixgrdio wrote
I see your point, but I would argue that no effect is still different from an effect which is functionally insignificant or outweighed by the inconvenience associated with a reduced speed limit.
[deleted] t1_ix8aicv wrote
Would that effect air quality much? Vehicles moving slower means they're releasing emissions in the area for longer.
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