seamustheseagull t1_ix7xl9k wrote
Reply to comment by SlightlyScruffy in A 20 mph speed limit intervention implemented at city centre scale had little impact on short- or long-term outcomes for road traffic collisions, casualties and speed. by _DeanRiding
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.
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