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).
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
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.
End to date: 060819, ams