Improvements in transportation can be categorized as:
a. Better cargo carrying characteristics.
b. More flexible routing.
c. Reduced real or perceived costs.
d. Higher speed or shorter movement time.
This
note is concerned with Transport Speed. The useable speeds have
increased dramatically in the last two centuries. By the 1960's increases in
possible speed began to peak out. The question is what may be practical
ultimate speeds for transportation on or near the earth's surface. Speeds
in SPACE are not included in this discussion.
The most
spectacular events in SPACE transport have been the manned trips to the Moon
that happened in the late 1960's. These were not particularly efficient transportation,
because there was very little of the vehicle systems recovered, and the fuel
expenditure was very large. Since that time more efficient space transport has
developed, but questions about the future remain.
Theoretical
maximum speeds are functions of tolerable accelerations and stage length. The upper limit near settled regions appears to be
limited by noise. So far no effective means of overcoming the noise
accompanying velocities above the speed of sound have been developed. For
the majority of trips the speed of sound can be considered the upper
practical limit. Operator capability limits practical speed.
Over time
systems are developed which help overcome human limitations and increase
safety. When human operators are involved then human capability will set upper
limits by sensory, response, comfort, strength and other limits of the effected
population. In recent years environmental considerations have begun to impose
types of limits that have not all been enunciated.
The limits as
described above can be applied to route design to minimize the possibility of
obsolescence due to speed restrictive alignment. There are valid reasons
for sinuous alignment, but in general the design speed used to choose the
roadway geometry should be somewhat above the anticipated immediate maximum.
This will increase safety. These comments apply to Highways which
include Roads and Streets, and also to other facilities such as Railways
or other fixed track systems.
There are
types of routes that accommodate the whole range of trip stage lengths such as Locals
and Collectors. Such facilities have built in intermediate stops and are
generally low speed.
Arterials
and Freeways have generally longer trip lengths and fewer stopping
opportunities. These are normally designed and operated at higher speeds .
An operator
or designer should understand the type of route under consideration and its
limitations.
Changes in
vehicle systems seem to have little effect on horizontal alignment
requirements. Therefore it is practical to align freeway and other trunk routes
such as express rail links in rural and urban areas for speeds as near the
practical maximum as possible. This will assure that the corridor that contains
the route will not become obsolescent for speed reasons even if the vehicle and
roadway system are replaced.
Theoretical
speed at any point on a trip stage varies according to the square root of the
distance between stops. The maximum speed at each location on the line can be
determined once the acceptable acceleration limits have been set.
Application
of the above principle requires curves of radius appropriate to the theoretical
speeds for the particular section where they are located. This approach will
encourage major changes in direction to the sections near vehicle stops.
There are
facilities such as FREEWAYS which have no intermediate stops or apparent
terminals. For such the alignment should be very good over the whole
intermediate length. The concept of high, nearly constant speed surface
transport trunk lines which bypass intermediate terminals with diverging and
merging to off line points is not new. The latest application on a large scale
to surface transport is the U.S. Interstate Highway System. The principle has
been applied to routes since the first long haul rail lines were constructed.
Generally
the good alignment on long haul routes has happened because the terrain allowed
it. Only in recent years since bridging, earthwork and tunneling become more
economical have high speed routes in difficult terrain become accepted.
Air routes
are similar but allow much greater speed variation on the same track by
vertical separation.
Since weight
is a form of acceleration the classical laws of mechanics suggest that some of
the apparent acceleration effects can be overcome by utilizing the gravity
vector to lessen the size of the apparent acceleration vector.
Super
elevation of curves utilizes the above principle and it can be extended to
increase the allowable line acceleration effects or to reduce the power
requirements for equivalent accelerations. Roller coasters and the vertical
profile used by the London Underground and other subways are examples of
utilizing these principles.
The
Underground uses two types, 'The Hump' which is close to the simple theoretical
shape for minimizing trip times, and 'The Saw Tooth' which is a later profile
developed from operational experience.
Ref. 4415; How The Underground Works.
The flight
path of an aircraft unfortunately is the reverse of what must be followed to
utilize the gravity vector to lower apparent acceleration.
For systems
like freeways, and proposed fixed route guide way systems such as GAITS
which operate on fixed speed trunk route concept, the maximum design speed has
to date been ultimately limited by the available vehicle power. Power unit
capability is subject to technological improvement. Unit weight, fuel
efficiency, durability, cost, emissions, etc. continue to improve and in many
cases the limits are not yet defined.
New routes
should have basic design speeds as high as practical to anticipate future
improvements. This is a reasonable approximation of current practice.
Establishment
of the criteria for setting the minimum design speeds for a particular route is
required to avoid potential obsolescence form low speed. Alignment for high
speed is usually more expensive than for low. The result is that alignment
costs have been a prime determinant of final route design speed.
The
principle of design speed uniformity along a particular route is well
established as a desirable situation. It is a means of utilizing vehicle
performance capability, improving safety, and maintaining a degree of
uniformity of line capacity.
The eventual
efficiency of a route will be a combined function of speed, capacity and
alignment cost the optional design speed requires analysis for each case.
Capacity loss at operating speeds below 50 km/h (30 mph.) for streets and
highways is well established. This should be the minimum design speed for all
but exceptional cases.
Each mode
has optimum and upper limits which effect capacity, safety, economy, etc.
Precise determination of these limits may not be possible but good forecasts of
their probable values are available. A comparison of the power requirements of
various modes for various speeds is shown in a Chart by Gabriella and Von Karmen.
Similar
relationships can be developed for some other speed determinants all of which
suggest that there are few potential surprises ahead from new hardware. The
variables which defy easy quantification are the social ones such as economy
which is highly dependent on the value of time, safety which is dependent on
the value of injury and life, and nuisance values such as noise, comfort, and
environmental concerns.
It is
apparent that conventional systems have much greater speed capability than is
presently utilized. Some of the limits are imposed by prior track design
decisions. Such limitations can be minimized in the future. The other
limitations are in terms of the current state of the art and the factors
mentioned above.
Aircraft
have shown the greatest gains in recent years and ultimate useful speeds have
been approached for short stage lengths. Fixed rail systems seem to have
potentials 2 or more times that which is generally utilized. Sustained motor
vehicle speeds of over 100 mph are now possible and may in fact be practicable.
It is difficult at this time to visualize the value of motor vehicle speeds
above 200 km/h (125 mph).
Major safety
problems begin to arise at high speeds due to the mix of trip lengths on
typical FREEWAYS. Several solutions to this type of problem have been
attempted. A good example is the express and local sections applied in the
north-south subway lines in
The
separation of trip length principle was extended to the multiple lane
configuration of Highway 401 in the
At the
present time the optimal approach to capacity and safety appears to have all
vehicles traveling at the same speed. Since this is almost achieved on FREEWAYS
that are running near optimal density there is little that can be done as long
as the vehicles are individually controlled by humans.
Some
advocates of 'ITS' (Intelligent Transportation Systems) see improvements by
better control and communication systems between vehicles and with the roadway.
Experiments with automatically controlled highway vehicles have been ongoing
for several decades.
The most
successful application has been cruse control that relies on either low
traffic density or the vehicles in the traffic stream traveling near the same
speed. If cruse control is combined with reliable forward object sensing then a
higher level of mixed speeds could be accommodated.
It has been
pointed out that the useful maximum speeds are related to trip (stage) length
which suggests that the trip length pattern will suggest the maximum useful
design speed. Since the corridor or right of way may have a longer useful life
than the original or current system.
Most route
alignments should therefore allow for as high a design speed as practical to
provide for possible changes in the systems that may use them in the future.
The current use of old roadway built for pedestrian and animal drawn vehicle
speeds illustrates the difficulties that can be encountered.
Note 15 discuss
some of the technologies used for transport. Each of the current modal types tends
to have limiting speeds. Speed also requires power and energy. The available
engine technology has traditionally provided limits to achievable speed.
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