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In a document dated 8/24/99, the City summarized the travel speeds on Bellevue Avenue before and after installation of the all-way stop signs at the corner of Bellevue & Fauquier in March 1999. The report concludes
Overall travel speeds have changed very little in the Bellevue corridor since the all-ways stops were installed.
In fact, the data contradict that conclusion.
They measured speeds on Bellevue between Lamont and Newport and again between Monticello and Mount Vernon. Their data don't show where in these blocks the counters were. Mr. Anderson says they generally place the counters at mid-block. Assuming that much, the location of the counters gives the first clue to how they have loaded the dice. On the map below, "S" marks the new stop signs, and the two "X" marks denote the midpoints of the blocks of the counters.
For sure, a driver at the location of the west counter on Bellevue between Monticello & Mt. Vernon would not be able to see the stop signs:
A driver on Bellevue at the location of the east counter between Lamont and Newport would be able to see the warning sign ca 500 ft. away if he knew where to look:
Yet we know from the literature (the Institute of Transportation Engineers treatise "Traffic Calming, the State of the Practice," Chapt. 5 at 119-20) that:
[W]hile [STOP signs'] impact on speed is limited to the immediate vicinity of intersections, in this respect they differ only in degree from any traffic calming measure, all of which have limited areas of influence. . . .
Traffic calming effects [of STOP signs] were found to be very localized, extending no farther than 150 to 200 feet downstream of intersections and even shorter distances upstream.
Thus the Richmond people have measured the speeds at three to five times the distance any effect would be expected. And they call this an experiment to find the effect of the signs!
Then the city reports the average of the incoming and outgoing speeds before and after at the two stations. They say the median decreased by 7.4%, and, apparently from that, they conclude "very little" effect.
Aside from the location of the counters there were two problems with that: The City calculated the % Change wrong and they had some of the median speeds wrong. Here, for what it is worth, are the data after they got the arithmetic fixed:
Data Before
Data After
DIR
Street
Between
Date
50th
Percentile85th Percentile
Date
50th Percentile
85th Percentile
Percent Change
50th %
85th %
EB
Bellevue
Monticello & Mt Vernon
3/18/99
29.9
35.3
8/11/99
28.2
33.8
-5.6
-4.1
WB
Bellevue
Monticello & Mt Vernon
3/18/99
30.2
35.4
8/11/99
28.9
34.6
-4.3
-2.2
EB
Bellevue
Lamont & Newport
3/18/99
31.4
37.2
8/11/99
26.7
31.7
-14.9
-14.9
WB
Bellevue
Lamont & Newport
3/18/99
25.9
30.9
8/11/99
26.6
32.6
2.7
5.5
Avg
3/18/99
29.3
34.7
8/11/99
27.6
33.2
-5.5
-3.9
(With the arithmetic done correctly the 7.4% decrease becomes a 5.5% decrease; it would have been in the City's interest to do the arithmetic right the first time).
It's no surprise that the data show slight changes approaching the signs from a distance where the ordinary driver won't even see the signs. More interesting are the changes departing the signs: The signs produced a 15% decrease in the median speed of the eastbound traffic, ca. 700 feet east of the signs, and a 4.3% decrease in the westbound speeds, ca. 1000 feet west the signs. For sure we have an eastbound effect at a distance some three times the distance at which the literature suggests there should be no effect.
The City attempted to wash out even that effect: They averaged the incoming change with the outgoing change. You'll recall that the ITE monograph says that stop signs have a less pronounced effect inbound than outbound. Here the City placed the counters at a distance where the average inbound driver won't even see the signs and then they averaged the inbound change in speed after the signs were installed with the outbound, with the result that they produced an average lower than the (unexpected, at that distance) change in the outbound speeds.
In short they designed an experiment that should not show any effect, and then they averaged the data that showed an effect with other data expected to show even less effect. Not surprisingly they then concluded that " travel speeds have changed very little."
For those of you who are curious, we analyze the data below. The graphs show the speed interval value reported by the City's counters; these appear to be the bottom end of the 5mph intervals. Note also that the curves below were furnished by Excel and they always peak at the highest datum, not necessarily where a fitted curve would peak.
Looking first at the East (Newport/Lamont) station, we notice essentially no effect upon the traffic driving west (toward the stop signs) and, as mentioned above, a very nice reduction in the speeds leaving the stop signs (note: 18 March was the day before they installed the All-ways):
These data are remarkably similar to the results Portland got with speed bumps:
Given the Richmond City folks' distaste for stop signs, we may want to discuss speed bumps with them.
The West station (Monticello/Mt. Vernon) is farther from the stop signs and it collects the left turns from MacArthur Ave. The data there show a slight reduction in speed toward the stop signs (!) and a slightly larger reduction in speed away from the new signs. Hard to figure, but there are the data:
After reviewing those data, the kindest thing we can say about the City's conclusion is that they are wrong. It doesn't take a terribly suspicious nature to conclude they are cooking the data.
Looking at these data another way, here are the east station (Lamont/Newport) data before and after the signs:
And here are the Monticello/Mt. Vernon data, also before and after:
In both cases it's hard to deny that something good happened. If they had measured closer to the signs we can bet the effect would have been clearer.
BTW: Please notice that none of these data shows the speeding up phenomenon that so worried Mr. Rhudy. Indeed, the only conclusion from these data is that the signs helped, and more signs might well be more helpful.
Here for completeness are the data from April 6, 1992, collected between Chevy Chase and Mt. Vernon, eastbound, and between Lamont and Newport, Westbound.
The Lamont/Newport data look much like the later data. The Chevy Chase/Mt. Vernon location is different, and just 1.5 blocks from the light at Hermitage. The traffic, nonetheless, is moving briskly. Not much different from the March 18 eastbound data at Monticello/Mt. Vernon
In any event, it's clear that the all-way stops at Bellevue and Fauquier have had a salutary effect.
The City thus took its data far past the distance where you would expect to see an effect. Then to minimize the effect they averaged in the data that should (and did) show even less effect. Even these cooked numbers show an improvement. Moreover, you don't need numbers to know that you now can turn left from MacArthur onto Bellevue without risking life, limb, and car.
It's time to have some more signs at Mount Vernon and, perhaps, at MacArthur. Maybe even at Crestwood. Call Chuck if you would like to work on this issue.
You also might be interested to see that more traffic leaves the neighborhood on Bellevue Ave. than enters.
Map courtesy
Last updated 02/24/02
Please send questions or comments to John Butcher