Hourly
volumes and average speed on I-805
The
top panel below shows total hourly volumes (in numbers of vehicles)
on I-805 through the Golden Triangle area (north of Governor Drive)
for a typical one week period (January 23-29, 2005). Blue curves
are for northbound traffic; red curves are southbound. These data
very clearly show the difference between Saturdays (far right hump),
Sundays (far left hump), and weekdays (middle five humps). The weekday
traffic shows prominent peaks for northbound morning traffic and
southbound afternoon traffic, due to commuter patterns. Northbound
and southbound trips are almost balanced at all times on weekends.
Unlike the case for Interstate
5, there is more weekday traffic northbound than southbound!
(That is, the area under the blue curves is greater than the area
under the red curves.)
The
bottom panel shows average speeds (in miles per hour) as a function
of time of day, for the same one week period. Once again, blue is
northbound and red is southbound. During this particular week, there
were delays in the northbound commute during AM peak on every day,
as reflected in the reduction in average speed. However, southbound
congestion during PM peak is far worse -- the average speed of southbound
travelers dips to 30 mph for several hours at a time.
Why
is it that the congestion is worse for southbound flow during afternoon
peak, when the top panel clearly shows that there are more vehicles
traveling northbound during the morning peaks? How can there be
worse congestion when there is less traffic?? Veteran traffic engineer
Andy Schlaefli of Urban Systems Associates proffers the following
interpretation of these data: The Golden Triangle area is a large
employment center, therefore cars traveling through the area are
generally exiting the freeways in the morning and entering the freeways
in the afternoon. Exiting causes less "friction" than
entering, i.e. cars which get off the freeway cause less disturbance
to freeway traffic than cars which get on. Thus, even though the
volumes are higher for northbound traffic during the mornings (because
this these are mostly commuters exiting the freeway) they don't
provoke severe jams. During afternoon peak, when they get back on
the freeway to go back home, the friction from the on-ramps is considerable,
and the flow is disturbed, leading to impressive jams. This is also
why the ramp metering is so severe at Nobel Drive.
Why
is this relevant for Genesee Avenue traffic? Because many commuters
don't want to get stuck in bumper-to-bumper messes on I-805, they
opt for Genesee as a bypass. Hence, the
situation on Genesee vis-a-vis directional volumes
is reversed: there are more PM southbound trips than AM northbound
trips on Genesee. This tells us that Regents Road, which is to the
west of Genesee, would provide a poor bypass, since most of these
trips are directed to the south and southeast. It also tells us
that relieving congestion on Genesee is like digging a hole at the
seashore - any traffic drawn off Genesee by the bridge is bound
to be replaced by new trips diverting from the 805 freeway.
|
