[sustran] becaks-reaction to Dr. Cervero

[Obwon]

Institute for Transportation and Development Policy (by way of
tkpb at barter.pc.my (Paul Barter)) wrote:
> 

[...]

  The traffic is such in Jakarta that
> nobody is traveling more than 5 or 10 km an hour-roughly the speed of a
> becak.  Travel speeds suffer from too many vehicles with too few passengers
> consuming too much road space, and only marginally by differences in vehicle
> travel speeds.   The problem in high-density Java is that the population
> density is too high to support much further extension of space-intensive
> auto travel, and the failure to allocate street space in a way which
> facilitates the travel of buses, paratransit, and non-motorized modes all of
> which consume less street space than the taxis and cars which clog the
> streets.
[...]

> Rgds,
> Walter Hook
> ITDP

-----------------------------

  This part of the article gave me pause to think -- How is the space, a
car or other motorized vehicle, calculated?

  We know that as cars speed up they need more room, about an additional
car length for each 10 m.p.h. of speed.  That's only a minimum standard
for drivers in good health with good eyesight and good reflexes.  It's
entirely likely that under normal conditions motorists will take even
more room.

  When a pedestrian crosses the street (here in NY jaywalking is common
for a number of reasons with I won't discuss here) they will often wait
even though the oncoming car is  still many dozens of seconds away, in
excess of the time needed to cross the 8ft lane the car will require.
Often this is due to caution because the car can easily speed up very
quickly so you can't simply judge or rate your chances to cross based on
the current observable speed of the oncoming car.

  At roughly 10ft per car length, at 30mph each car then takes up 10+ 30
or 40 ft.  or 132 cars per mile.  But at 40mph we get 105.6 cars per
mile.  A loss of road capacity of 26.4 cars.  This may very well be why
small obstructions, like double parked cars or cars entering or leaving
the roadway produce dramatic interferences with traffic flow.  So if a
roadway presents many opportunities for cars to obstuct its flow, such
as a street which has parking on either side, it needs more space to
allow for the expected obstructions to occur, or it's flow potential
calculation must be reduced.

   A street that passes many stores, as in a commercial district, must
be assumed to accommodate deliveries as well as people who will stop to
shop.  Often, I note, that many of the parking spaces in these
commercial areas, are taken up, not by shoppers or delivery vehicles,
but by local residents who get first crack at the spaces.  So delivery
vehicles are forced to double park and thus reduce the number of cars
per mile that the road has the capacity to serve.

  Are these temporary obstructions included in calculating how much
traffic a road can handle?  Like on highways, where there can be
expectations of cars breaking down or accidents.  If there are expected
numbers of accidents per no. of vehicles, then this must have impact on
the amount of traffic that the highway can be expected to handle.  At 60
mph a car needs 70 ft. clear of traffic and obstructions to operate.  Or
75.43 cars per mile.  This is reduced by the number of access/exit ramps
and also by the expected number of accidents expected to occur.  

  But I'm given to suspect that city street capacities don't come
anywhere near what they are being calculated to carry.  Simply because I
think they are calculated without including even normally expected
obstructions like deliveries, double parking and cars entering and
leaving parking spaces.  I suspect that if downtown, urban road
capacities include provisions for, obstructions expected to take place,
plus a figure for unexpected obstructions, roadway capacity would fall
to where the need for alternative resources would be more clear.

  Obwon