Evening star. [volume] (Washington, D.C.) 1854-1972, 1931-02-15
BY M. O. ELDRIDGE,

DI IVA. Os ELDhAOO6S
Assistant DsectSE Of TTas and SHsiTman
American Road Builders Association.
Various formulas have been developed
to determine the maximum theoretical
capacities of a single lane of trafic.
The principal factor to be considered
in the development of such formulas
is the distance which must be main
tanned between cars moving at various
speeds.
This distance is controlled by several
factors, chief among which is the per-
ception time Of the driver which may
be defined as the time required to per-
ceive that the preceding car is going
to slow down or stop, plus the reaction
time of the driver.
If all cars could be stopped in the
same distance and if the reaction time
of all drivers were the same and all
types Of pavement had the same coefa-
cent of friction, the problem would be
comparatively simple. A skillful and
experienced driver usually assumes that
the car ahead of him will stop in
case of emergency in the same distance
as his own car and that he will not
stop suddenly without warning unless
the object for which he stops is
observed by both drivers at the same
time.
Stop Signal Beneficial.
The stop" signal on the rear of
the modern automobile has done as
much as any other one, thing to in
crease the capacity Of highways, for it
enables a driver, when confronted by
an emergency, to indicate to the follow
ing drivers or drivers that he should
stop or slow down.
Unfortunately, all drivers are not
equally alert and skillful nor will all
cars stop within the same distance
from the same speed, even on the
same type Of pavement. For these
reasons the factors used in developing
capacity formulas have an infinite
variety of values obtained for various
assumptions of the variables involved.
It is obvious that in slow-noving
traffic cars may be safely driven closer
together than in fast traffic. AS the
speed increases the distance between
the cars must increase. This distance
will vary somewhat in proportion to
the speed.
In practice the distance in feet
amounts to about twice the speed in
miles per hour, although some engineers
| contend that the distance should be the
| number of feet that the cars are NOV
ing in one to one and one-half seconds.
This, they contend, is the time required,
under ordinary circumstances, to slow
down the following car, or cars, in
order to avoid rear-end collisions.
1t does not follow, therefore, that
the capacity Of street increases In
proportion to the speed. Maximum
capacity formulas indicate that the
traffic capacity of street or road is
relatively greater at speeds under IS
miles per hour than at greater speeds.
The maximum theoretical capacity
of a single lane of motor vehicles
appears to be approximately 1900 We
hicles an hour. This figure is based
on the assumption that the vehicles
are moving at uniform speed and will
maintain a uniform spacing between
each other and that the flow will not
be interrupted nor interfered with by
turning or crossing traffic.
1t might be assumed that the nearest
! approach to this condition exists in the
Holland vehicular tunnels. In order to
obtain a record of traffic moving in
a single lane under these conditions
a closeup observation of the tubes and
| their traffic capacities was made by the
writer on November IS and is, 1880.
The north tube is 1857 feet long,
portal to portal, while the south tube
is 1871 feet long. The roadways in
each of the tunnels are 20 feet wide
in their narrowest portions and carry
two lanes of traffic-one for slow-mov-
ing trucks and the other for fast
| moving passenger cars. Eastbound
traffic moves through the south tun-
mel and westbound traffic through the
north tunnel, with slow traffic on the
right and fast traffic on the left.
The lanes in each tube are separated
by a white painted line. Vehicles are
required to keep in line, rs feet apart,
, and to move at not more than 80 miles
an hour and not less than miles an
hour. Double-deck buses, horSe-draWn
vehicles, push carts, bicycles and pedes-
TRANS are prohibited from using the
| tunnel.
The exits and entrances of the tubes
are separated from each other by the
distance Of an ordinary city block in
order to prevent traffic congestion in
the streets in the immediate vicinity
of the tunnel
Incidently. the air in the tubes can
be changed in less than one and one
half minutes, or about AZ times an
hour. 1t is said that there is less
carbon monoxide in the air in the tubes
than on some of the streets which
extend between tall buildings.
On November 16. during the morning
rllSh-hour. our two trips were made
through the t1nnel-one round trip
through the fast lane and one round
trip following the trucks through the
slow lane.
BY means Of the Hasler Tel Recorder,
which records the time, speed and dis
lance, 1t was found that by following
line of passenger motor vehicles about