[sustran] Re: Fw: Fw: Re: Traffic Congestion in Bangalore
(WITHATTACHMENT)
Chris Bradshaw
c_bradshaw at rogers.com
Sat Mar 22 05:33:56 JST 2008
Counting trips on foot is indeed a challenge, one that most traffic
engineers don't worry about resulting in undercounts.
I am aware of four different types.
1. The origin-destination survey, used to feed data to computer models, uses
telephone surveys to ask people what mode is used for different trips. The
first problem is the fallibility of memory, and the second a certain
political correctness that can cause respondants to be less than truthful.
And thirdly, there is the sampling used in choosing a sample, or the trips
they query about, or the date for the survey, etc. And the coding, as was
mentioned, doesn't allow for multi-mode trips to be recorded as such. For
instance, I estimate that transit trips are made up of 50% of the time
duration to be in walking and waiting, for buses and don't-walk signals.
2. The second is cordon counts, in which the mix of travelers is recorded
across a particular line on a map. These lines are chosen for simplicity,
where the traffic patterns are least complex, e.g., green corridors, rail
lines, rivers/bridges, etc. Not only do people on foot avoid trips across
such empty spaces, but walking trips across these are not going to be long
enough to cross a second cordon in the same time vs trips by car. Walking
is undercounted, in part, because the trips are shorter.
3. The third is the intersection count. The vehicular modes are all counted
simply, but pedestrians are problematic. Those turning right, and therefore
not leaving the sidewalk, are not counted at all. But, those who have to
use two intersecting crosswalk to complete their trip might be counted
twice.
Of course, trips using only local streets, by any mode, are rarely counted
at all, partly because these streets are rarely used that much. Only when
neighbours complaint about "cut-through traffic" do cities count them, and
then they ignore walking, although the presence of pedestrians is relevant,
but in the opposite way: pedestrians represent both the general positive
presence of "eyes on the street,": (a la Jane Jacobs) and become part of the
'traffic calming'; but also a measurement of the human endangerment that
such traffic poses, not to mention a measurement of how bad the car problem
is, since the more pedestrians, the less serious the perceived threat.
4. Finally, there is the matter of "warrants" for pedestrian crossings.
Here traffic engineers count the number of pedestrians only that _attempt_
to cross a roadway at a particular section. The higher the count, the
greater the warrant (the justification) for providing a special combination
of paint lines and perhaps a signal or extra lighting to help pedestrians
cross. The problem with it, is that where such crossing assistance is most
needed is probably where few crossings are attempted and where pedestrians
long ago found other routes or, equally likely, other modes of travel.
Where there are plenty of people crossing is where little improvement is
needed, although often there can be problems with pedestians with special
needs.
Chris Bradshaw
Ottawa
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