Fundamentals of Transportation/Networks
Transportation systems have specific structure. Roads have length, width, and depth. The characteristics of roads depends on their purpose.
Roads[edit | edit source]
A road is a path connecting two points. The English word ‘road’ comes from the same root as the word ‘ride’ –the Middle English ‘rood’ and Old English ‘rad’ –meaning the act of riding. Thus a road refers foremost to the right of way between an origin and destination. In an urban context, the word street is often used rather than road, which dates to the Latin word ‘strata’, meaning pavement (the additional layer or stratum that might be on top of a path).
Modern roads are generally paved, and unpaved routes are considered trails in some countries. The pavement of roads began early in history. Approximately 2600 BCE, the Egyptians constructed a paved road out of sandstone and limestone slabs to assist with the movement of stones on rollers between the quarry and the site of construction of the pyramids. The Romans and others used brick or stone pavers to provide a more level, and smoother surface, especially in urban areas, which allows faster travel, especially of wheeled vehicles. The innovations of Thomas Telford and John McAdam reinvented roads in the early nineteenth century, by using less expensive smaller and broken stones, or aggregate, to maintain a smooth ride and allow for drainage. Later in the nineteenth century, application of tar (asphalt) further smoothed the ride. In 1824, asphalt blocks were used on the Champs-Élysées in Paris. In 1872, the first asphalt street (Fifth Avenue) was paved in New York (due to Edward de Smedt), but it wasn’t until bicycles became popular in the late nineteenth century that the “Good Roads Movement” took off. Bicycle travel, more so than travel by other vehicles at the time, was sensitive to rough roads. Demands for higher quality roads really took off with the widespread adoption of the automobile in the United States in the early twentieth century.
The first good roads in the twentieth century were constructed of Portland cement concrete (PCC). The material is stiffer than asphalt (or asphalt concrete) and provides a smoother ride. Concrete lasts slightly longer than asphalt between major repairs, and can carry a heavier load, but is more expensive to build and repair. While urban streets had been paved with concrete in the US as early as 1889, the first rural concrete road was in Wayne County, Michigan, near to Detroit in 1909, and the first concrete highway in 1913 in Pine Bluff, Arkansas. By the next year over 2300 miles of concrete pavement had been laid nationally. However over the remainder of the twentieth century, the vast majority of roadways were paved with asphalt. In general only the most important roads, carrying the heaviest loads, would be built with concrete.
Roads are generally classified into a hierarchy. At the top of the hierarchy are freeways, which serve entirely a function of moving vehicles between other roads. Freeways are grade-separated and limited access, have high speeds and carry heavy flows. Below freeways are arterials. These may not be grade-separated, and while access is still generally limited, it is not limited to the same extent as freeways, particularly on older roads. These serve both a movement and an access function. Next are collector/distributor roads. These serve more of an access function, allowing vehicles to access the network from origins and destinations, as well as connecting with smaller, local roads, that have only an access function, and are not intended for the movement of vehicles with neither a local origin nor destination. Local roads are designed to be low speed and carry relatively little traffic.
The class of the road determines which level of government administers it. The highest roads will generally be owned, operated, or at least regulated (if privately owned) by the higher level of government involved in road operations; in the United States, these roads are operated by the individual states. As one moves down the hierarchy of roads, the level of government is generally more and more local (counties may control collector/distributor roads, towns may control local streets). In some countries freeways and other roads near the top of the hierarchy are privately owned and regulated as utilities, these are generally operated as toll roads. Even publicly owned freeways are operated as toll roads under a toll authority in other countries, and some US states. Local roads are often owned by adjoining property owners and neighborhood associations.
The design of roads is specified in a number of design manual, including the AASHTO Policy on the Geometric Design of Streets and Highways (or Green Book). Relevant concerns include the alignment of the road, its horizontal and vertical curvature, its super-elevation or banking around curves, its thickness and pavement material, its cross-slope, and its width.
Freeways[edit | edit source]
A motorway or freeway (sometimes called an expressway or thruway) is a multi-lane divided road that is designed to be high-speed free flowing, access-controlled, built to high standards, with no traffic lights on the mainline. Some motorways or freeways are financed with tolls, and so may have tollbooths, either across the entrance ramp or across the mainline. However in the United States and Great Britain, most are financed with gas or other tax revenue.
Though of course there were major road networks during the Roman Empire and before, the history of motorways and freeways dates at least as early as 1907, when the first limited access automobile highway, the Bronx River Parkway began construction in Westchester County, New York (opening in 1908). In this same period, William Vanderbilt constructed the Long Island Parkway as a toll road in Queens County, New York. The Long Island Parkway was built for racing and speeds of 60 miles per hour (96 km/hr) were accommodated. Users however had to pay a then expensive $2.00 toll (later reduced) to recover the construction costs of $2 million. These parkways were paved when most roads were not. In 1919 General John Pershing assigned Dwight Eisenhower to discover how quickly troops could be moved from Fort Meade between Baltimore and Washington to the Presidio in San Francisco by road. The answer was 62 days, for an average speed of 3.5 miles per hour (5.6 km/hr). While using segments of the Lincoln Highway, most of that road was still unpaved. In response, in 1922 Pershing drafted a plan for an 8,000 mile (13,000 km) interstate system which was ignored at the time.
The US Highway System was a set of paved and consistently numbered highways sponsored by the states, with limited federal support. First built in 1924, they succeeded some previous major highways such as the Dixie Highway, Lincoln Highway and Jefferson Highway that were multi-state and were constructed with the aid of private support. These roads however were not in general access-controlled, and soon became congested as development along the side of the road degraded highway speeds.
In parallel with the US Highway system, limited access parkways were developed in the 1920s and 1930s in several US cities. Robert Moses built a number of these parkways in and around New York City. A number of these parkways were grade separated, though they were intentionally designed with low bridges to discourage trucks and buses from using them. German Chancellor Adolf Hitler appointed a German engineer Fritz Todt Inspector General for German Roads. He managed the construction of the German Autobahns, the first limited access high-speed road network in the world. In 1935, the first section from Frankfurt am Main to Darmstadt opened, the total system today has a length of 11,400 km. The Federal-Aid Highway Act of 1938 called on the Bureau of Public Roads to study the feasibility of a toll-financed superhighway system (three east-west and three north-south routes). Their report Toll Roads and Free Roads declared such a system would not be self-supporting, advocating instead a 43,500 km (27,000 mile) free system of interregional highways, the effect of this report was to set back the interstate program nearly twenty years in the US.
The German autobahn system proved its utility during World War II, as the German army could shift relatively quickly back and forth between two fronts. Its value in military operations was not lost on the American Generals, including Dwight Eisenhower.
On October 1, 1940, a new toll highway using the old, unutilized South Pennsylvania Railroad right-of-way and tunnels opened. It was the first of a new generation of limited access highways, generally called superhighways or freeways that transformed the American landscape. This was considered the first freeway in the US, as it, unlike the earlier parkways, was a multi-lane route as well as being limited access. The Arroyo Seco Parkway, now the Pasadena Freeway, opened December 30, 1940. Unlike the Pennsylvania Turnpike, the Arroyo Seco parkway had no toll barriers.
A new National Interregional Highway Committee was appointed in 1941, and reported in 1944 in favor of a 33,900 mile system. The system was designated in the Federal Aid Highway Act of 1933, and the routes began to be selected by 1947, yet no funding was provided at the time. The 1952 highway act only authorized a token amount for construction, increased to $175 million annually in 1956 and 1957.
The US Interstate Highway System was established in 1956 following a decade and half of discussion. Much of the network had been proposed in the 1940s, but it took time to authorize funding. In the end, a system supported by gas taxes (rather than tolls), paid for 90% by the federal government with a 10% local contribution, on a pay-as-you-go” system, was established. The Federal Aid Highway Act of 1956 had authorized the expenditure of $27.5 billion over 13 years for the construction of a 41,000 mile interstate highway system. As early as 1958 the cost estimate for completing the system came in at $39.9 billion and the end date slipped into the 1980s. By 1991, the final cost estimate was $128.9 billion. While the freeways were seen as positives in most parts of the US, in urban areas opposition grew quickly into a series of freeway revolts. As soon as 1959, (three years after the Interstate act), the San Francisco Board of Supervisors removed seven of ten freeways from the city’s master plan, leaving the Golden Gate bridge unconnected to the freeway system. In New York, Jane Jacobs led a successful freeway revolt against the Lower Manhattan Expressway sponsored by business interests and Robert Moses among others. In Baltimore, I-70, I-83, and I-95 all remain unconnected thanks to highway revolts led by now Senator Barbara Mikulski. In Washington, I-95 was rerouted onto the Capital Beltway. The pattern repeated itself elsewhere, and many urban freeways were removed from Master Plans.
In 1936, the Trunk Roads Act ensured that Great Britain’s Minister of Transport controlled about 30 major roads, of 7,100 km (4,500 miles) in length. The first Motorway in Britain, the Preston by-pass, now part of the M-6, opened in 1958. In 1959, the first stretch of the M1 opened. Today there are about 10,500 km (6300 miles) of trunk roads and motorways in England.
Australia has 790 km of motorways, though a much larger network of roads. However the motorway network is not truly national in scope (in contrast with Germany, the United States, Britain, and France), rather it is a series of local networks in and around metropolitan areas, with many intercity connection being on undivided and non-grade separated highways. Outside the Anglo-Saxon world, tolls were more widely used. In Japan, when the Meishin Expressway opened in 1963, the roads in Japan were in far worse shape than Europe or North American prior to this. Today there are over 6,100 km of expressways (3,800 miles), many of which are private toll roads. France has about 10,300 km of expressways (6,200 miles) of motorways, many of which are toll roads. The French motorway system developed through a series of franchise agreements with private operators, many of which were later nationalized. Beginning in the late 1980s with the wind-down of the US interstate system (regarded as complete in 1990), as well as intercity motorway programs in other countries, new sources of financing needed to be developed. New (generally suburban) toll roads were developed in several metropolitan areas.
An exception to the dearth of urban freeways is the case of the Big Dig in Boston, which relocates the Central Artery from an elevated highway to a subterranean one, largely on the same right-of-way, while keeping the elevated highway operating. This project is estimated to be completed for some $14 billion; which is half the estimate of the original complete US Interstate Highway System.
As mature systems in the developed countries, improvements in today’s freeways are not so much widening segments or constructing new facilities, but better managing the road space that exists. That improved management, takes a variety of forms. For instance, Japan has advanced its highways with application of Intelligent Transportation Systems, in particular traveler information systems, both in and out of vehicles, as well as traffic control systems. The US and Great Britain also have traffic management centers in most major cities that assess traffic conditions on motorways, deploy emergency vehicles, and control systems like ramp meters and variable message signs. These systems are beneficial, but cannot be seen as revolutionizing freeway travel. Speculation about future automated highway systems has taken place almost as long as highways have been around. The Futurama exhibit at the New York 1939 World’s Fair posited a system for 1960. Yet this technology has been twenty years away for over sixty years, and difficulties remain.
Layers of Networks[edit | edit source]
|OSI Reference Model for the Internet|
|Data||7. Application||Network process to application|
|6. Presentation||Data representation,encryption and decryption|
|5. Session||Interhost communication|
|Segments||4. Transport||End-to-end connections and reliability,Flow control|
|Packet||3. Network||Path determination and logical addressing|
|Frame||2. Data Link||Physical addressing|
|Bit||1. Physical||Media, signal and binary transmission|
All networks come in layers. The OSI Reference Model for the Internet is well-defined. Roads too are part of a layer of subsystems of which the pavement surface is only one part. We can think of a hierarchy of systems.
- Trip Ends
- End to End Trip
- Service (Vehicle & Schedule)
- Signs and Signals
- Pavement Surface
- Structure (Earth & Pavement and Bridges)
- Alignment (Vertical and Horizontal)
At the base is space. On space, a specific right-of-way is designated, which is property where the road goes. Originally right-of-way simply meant legal permission for travelers to cross someone's property. Prior to the construction of roads, this might simply be a well-worn dirt path.
On top of the right-of-way is the alignment, the specific path a transportation facility takes within the right-of-way. The path has both vertical and horizontal elements, as the road rises or falls with the topography and turns as needed.
Structures are built on the alignment. These include the roadbed as well as bridges or tunnels that carry the road.
Pavement surface is the gravel or asphalt or concrete surface that vehicles actually ride upon and is the top layer of the structure. That surface may have markings to help guide drivers to stay to the right (or left), delineate lanes, regulate which vehicles can use which lanes (bicycles-only, high occupancy vehicles, buses, trucks) and provide additional information. In addition to marking, signs and signals to the side or above the road provide additional regulatory and navigation information.
Services use roads. Buses may provide scheduled services between points with stops along the way. Coaches provide scheduled point-to-point without stops. Taxis handle irregular passenger trips.
Drivers and passengers use services or drive their own vehicle (producing their own transportation services) to create an end-to-end trip, between an origin and destination. Each origin and destination comprises a trip end and those trip ends are only important because of the places at the ends and the activity that can be engaged in. As transportation is a derived demand, if not for those activities, essentially no passenger travel would be undertaken.
With modern information technologies, we may need to consider additional systems, such as Global Positioning Systems (GPS), differential GPS, beacons, transponders, and so on that may aide the steering or navigation processes. Cameras, in-pavement detectors, cell phones, and other systems monitor the use of the road and may be important in providing feedback for real-time control of signals or vehicles.
Each layer has rules of behavior:
- some rules are physical and never violated, others are physical but probabilistic
- some are legal rules or social norms which are occasionally violated
Hierarchy of Roads[edit | edit source]
Even within each layer of the system of systems described above, there is differentiation.
Transportation facilities have two distinct functions: through movement and land access. This differentiation:
- permits the aggregation of traffic to achieve economies of scale in construction and operation (high speeds);
- reduces the number of conflicts;
- helps maintain the desired quiet character of residential neighborhoods by keeping through traffic away from homes;
- contains less redundancy, and so may be less costly to build.
|Functional Classification||Types of Connections||Relation to Abutting Property||Minnesota Examples|
|Limited Access (highway)||Through traffic movement between cities and across cities||Limited or controlled access highways with ramps and/or curb cut controls.||I-94, Mn280|
|Linking (arterial:principal and minor)||Traffic movement between limited access and local streets.||Direct access to abutting property.||University Avenue, Washington Avenue|
|Local (collector and distributor roads)||Traffic movement in and between residential areas||Direct access to abutting property.||Pillsbury Drive, 17th Avenue|
Model Elements[edit | edit source]
Transportation forecasting, to be discussed in more depth in subsequent modules, abstracts the real world into a simplified representation.
Recall the hierarchy of roads. What can be simplified? It is typical for a regional forecasting model to eliminate local streets and replace them with a centroid (a point representing a traffic analysis zone). Centroids are the source and sink of all transportation demand on the network. Centroid connectors are artificial or dummy links connecting the centroid to the "real" network. An illustration of traffic analysis zones can be found at this external link for Fulton County, Georgia, here: traffic zone map. Keep in mind that Models are abstractions.
Network[edit | edit source]
- Zone Centroid - special node whose number identifies a zone, located by an "x" "y" coordinate representing longitude and latitude (sometimes "x" and "y" are identified using planar coordinate systems).
- Node (vertices) - intersection of links , located by x and y coordinates
- Links (arcs) - short road segments indexed by from and to nodes (including centroid connnectors), attributes include lanes, capacity per lane, allowable modes
- Turns - indexed by at, from, and to nodes
- Routes, (paths) - indexed by a series of nodes from origin to destination. (e.g. a bus route)
- Modes - car, bus, HOV, truck, bike, walk etc.
Matrices[edit | edit source]
Scalar[edit | edit source]
A scalar is a single value that applies model-wide; e.g. the price of gas or total trips.
Vectors[edit | edit source]
Vectors are values that apply to particular zones in the model system, such as trips produced or trips attracted or number of households. They are arrayed separately when treating a zone as an origin or as a destination so that they can be combined into full matrices.
- vector (origin) - a column of numbers indexed by traffic zones, describing attributes at the origin of the trip (e.g. the number of households in a zone)
|Trips Produced at Origin Zone|
|Origin Zone 1||Ti1|
|Origin Zone 2||Ti2|
|Origin Zone 3||Ti3|
- vector (destination) - a row of numbers indexed by traffic zones, describing attributes at the destination
|Destination Zone 1||Destination Zone 2||Destination Zone 3|
|Trips Attracted to Destination Zone||Tj1||Tj2||Tj3|
Full Matrices[edit | edit source]
A full or interaction matrix is a table of numbers, describing attributes of the origin-destination pair
|Destination Zone 1||Destination Zone 2||Destination Zone 3|
|Origin Zone 1||T11||T12||T13|
|Origin Zone 2||T21||T22||T23|
|Origin Zone 3||T31||T32||T33|
Thought Questions[edit | edit source]
- Identify the rules associated with each layer?
- Why aren’t all roads the same?
- How might we abstract the real transportation system when representing it in a model for analysis?
- Why is abstraction useful?
Abbreviations[edit | edit source]
- SOV - single occupant vehicle
- HOV - high occupancy vehicle (2+, 3+, etc.)
- TAZ - transportation analysis zone or traffic analysis zone
Variables[edit | edit source]
- msXX - scalar matrix
- moXX - origin vector matrix
- mdXX - destination vector matrix
- mfXX - full vector matrix
- - Total Trips
- - Trips Produced from Origin Zone
- - Trips Attracted to Destination Zone
- - Trips Going Between Origin Zone and Destination Zone
Key Terms[edit | edit source]
- Zone Centroid
- Pavement Surface
- Signs and Signals
- End to End Trip
- Trip Ends
External Exercises[edit | edit source]
Use the ADAM software at the STREET website and examine the network structure. Familiarize yourself with the software, and edit the network, adding at least two nodes and four one-way links (two two-way links), and deleting nodes and links. What are the consequences of such network adjustments? Are some adjustments better than others?