A friend had an incident near where he lives recently that is similar to my line of work (currently) that made me think what constitutes a significant drop in speed as a result of a change in road dimensions?
In short, the friend lives in a small village and there had been problems of people speeding through it (predominantly east to west – they get through traffic mostly in one direction). The local authority did a speed survey to the east and west of the village, narrowed the road in the village and then did three surveys afterwards (to the east of the village as a direct comparison, at the new measure in the village and to the west again as a direct comparison).
They found that traffic had slowed by 4 miles per hour on average on the approach to the east of the village (40mph limit), by comparing the results of the two, 7 day long surveys (the average on most days was 3.5mph – 4.5mph slower). This obviously had a reduced 85th percentile figure as well (as the majority of traffic was going that bit slower).
The thought being that drivers anticipated the road narrowing once they were aware of it and slowed down slightly (the survey was about 300m before the narrowing but in eyesight of the narrowing). The survey on the exit showed little change (less than 1mph as presumably drivers accelerated away from the measure and out of the village onto wider roads).
I think the other survey that they did at the measure was just to get an idea of the effect of the measure (e.g. what effect it had at that point, they were unsure of where they were going to site it originally so couldn't survey the exact spot beforehand).
What I wondered was if the 4mph reduction on approach was actually significant? It was obviously a noticeable drop (10% of the posted limit), but could it be explained by just a general variance in traffic speeds (e.g. could it be expected)? My thought was over a week this would be unlikely.
Apparently the traffic flows over the two survey weeks were similar – within 5% (so a reasonably fair comparison in that respect), and there were no outside events to effect the survey (e.g. inclement weather, roadworks and diversions etc.).
I just couldn’t figure how you could class what a significant reduction in speed would be in this case? I have done similar work myself (this kind of data usually comes split down into 5mph speed differentials – e.g. 0-5, 5-10, 10-15 etc.) so you can work out the means, 85th percentile’s etc.
Most of the time, I will just take a quick look at the differences before or after in the mean average and 85th percentile as it accounts for the bulk of traffic and make my judgment from there (and only dig further if I need to which is a rare occasion), but it did just make me think what would be a statistically significant drop?