DESCRIPTION OF THE
OAK RIDGE NATIONAL HIGHWAY NETWORK
Abstract / Overview / Sources / Construction / Locational Accuracy / Topological Accuracy / Network File Structure / Node File / Link file / Link Data Element Description / Compatibility / Acknowledgements
Version 1.4 (C) (Complete)
Center for Transportation Analysis, Oak Ridge National Laboratory
October 2, 1996 Revised January 18, 2004
The Oak Ridge National
Highway Network (NHN) is a database of major highways in the United
States. It succeeds the National Highway Planning Network, version
1, with the same basic structure and format. It is designed primarily
to address vehicle routing and scheduling problems, but naturally
may be used in other studies that require an analytic or geographically-based
national highway network. The NHN includes both attribute and locational
data about roadways acquired from a wide variety of sources. It has
been enhanced at Oak Ridge National Laboratory by adding additional
roads and attribute detail and adjusting topology to produce a true
analytic network. This documentation is intended primarily to assist
users of this database by describing its structure, data elements,
The Oak Ridge National
Highway Network is a geographically based analytic network of the
major highways in the United States. It was developed at Oak Ridge
National Laboratory to support analyses of a wide variety of highway
transportation issues that require use of a network. Potential uses
include studies of highway performance and network design, vehicle
routing and scheduling, and social and environmental impacts of transportation.
It presently contains approximately 500,000 centerline miles of roadway
and will, with varying degrees of accuracy, show the location of
these roads and attribute detail about their characteristics. Although
it includes many roads of lower class, it may be thought of fundamentally
as an arterial network. The ultimate intent is to represent all rural
arterials and most urban principal arterials, but not collectors
or urban minor arterials unless they are part of through highways.
Data included in this network come from several sources. All data has been derived from or checked against information obtained from state and Federal governmental agencies.
(1) USGS National Atlas Digital Line Graphs
The original foundation of the NHN was the set of roadways digitized by the U.S. Geological Survey (USGS) from 1:2,000,000 scale plates of the National Atlas, circa 1980. Very few of these shapes remain, and all have been rectified to some degree.
(2) State Maps
highway maps ("tourist maps") are the primary source for
most data on the physical characteristics of roads. They are also
the primary means to update the network with recently opened roads.
The typical state highway map shows at least all state highways,
indicates distances between major junctions, has urban area inserts,
and was published by a state agency.
(3) Other Maps
We had county road maps and highway district maps from a number of states, which were used for whatever detail they could supply, topological and attribute. USGS 1:250,000 and 1:100,000 scale maps were frequently consulted to resolve locational and attribute details. We considered them supplemental sources to be used when the state maps were inconclusive or not detailed enough.
FHWA allowed us access to the Highway Performance Monitoring System (HPMS), which is logically an inventory of highways in the United States supplied by individual state highway agencies (SHAs). In many states it is possible to translate between individual records and the real roadways they describe. When it was possible, selected attribute information in the HPMS was transcribed to network links.
(5) Defense Movement Coordinators
One of the functions of this database is to assist military personnel in the management of military movements. State National Guard offices have Defense Movement Coordinators (DMCs) who are familiar with their own states' highway systems and who maintain contact with both state highway agencies and local units. Because of their management responsibilities, they have an interest in correcting errors and maintaining the accuracy and currency of the data.
(6) USGS 1:100,000 Digital Line Graphs
For roads represented in Version 1 of the NHPN, geographic shapes were extracted from the USGS DLGs in most states, provided the roads were represented. That is, attributes and topology were unchanged while the cartography was replaced. This replacement was conducted by the University of Tennessee Transportation Center (UTTC).
(7) TIGER/Line Files
For roads not represented in the DLGs, and for editing work subsequent to 1993, road locations were often extracted from the Census Bureau's TIGER/Line files.
(8) Digitization of Urban Area Maps
States provided FHWA with maps of urbanized areas that highlighted principal arterials. UTTC digitized these roads. They are the most common source of location and functional class for roads not in the original NHPN.
(9) NHPN, version 2
Subsequent to 1993, FHWA continued development of their version of the National Highway Planning Network, known as version 2. This data has also been used as a source, primarily for geographic shapes, functional classes, and National Highway System flags which had not yet been incorporated into the NHPN version 1 at the time the NHN was established.
(10) Canada and Mexico
Matchstick networks for Canada and Mexico have also been added, originally developed by Dave Clarke of UTTC. We have tried to make attributes reasonably consistent with the US portion, so that identical models may be used with the unified network.
The base data for this highway network were the 1:2,000,000 USGS digital line graphs. The locational data were edge matched, scissored into state subnetworks, and element identifications were assigned. As later described, initial geographically calculated distances were assigned to each link. In addition, other attribute details that could be deduced from the USGS attributes (e.g., most sign routes, access control, and divided highway flags) were also incorporated.
An initial clean-up pass was made through every state to check or add sign routes and check distances. The most serious topological problems were also resolved at this time. Except for location, attribute editing was done with a text editor. For location, a set of programs were locally written to move links and junctions, adjust their shapes, subdivide original links by establishing new junctions along their length, add new non-digitized (straight line) links between nodes, and incorporate digitized chains from other sources. At this stage, new junction locations were estimated visually, either relative to an existing link or at a free-standing new absolute location. Initially, the network contained approximately 320,000 miles.
In the second editing phase, many new links were added because they were required as potential military convoy routes. We also attempted to include in the network the entire Federal-Aid Primary system (essentially all rural arterials, though only a small proportion of urban arterials). Again, new link locations were not digitized, although more care was taken in the location of new junctions. During this second pass through the states, road locations were compared to large scale USGS maps and the more egregious problems were corrected, so many of the road alignments were changed from the original DLGs. Insofar as our data sources permitted, we attempted to add administrative and functional class indicators to network links.
The third phase was the incorporation of 100K DLG alignments to improve geographic accuracy to 100 m, the incorporation of all urban principal arterials, and the reclassification of roadway functional classes.
Users must recognize that geographic accuracy varies. In fact, of all the design objectives in the construction of an analytic network like the NHN, cartographic consistency is probably the lowest. Source codes for individual links will at least approximately reflect accuracy levels, and should make possible not only the identification of substandard links, but also their automated removal for cartographic applications when consistency is desired.
There are several factors a user should consider that degrade geographic accuracy:
(a) Centerline representation. In the case of divided roadways, the ideal was to have a roadway shape located in the middle of the median. In fact, this was rarely achieved. Usually, when separated directional roadways were far enough apart to have distinct alignments in the DLGs, one of those roadways (invariably the shortest) was chosen to represent both directions of travel. In the case of couplets following one-way city streets at least a block apart, however, the NHN design standard is to have distinct links for the two directions of travel.
(b) Interchange representation. Note that the dimensions of a typical interchange substantially exceed the target accuracy for the network. But for reasons of source data availability, editing cost, and the requirements of most of our application programs, interchanges and traffic circles are almost always represented by a single point, or node. Individual ramps are explicitly included in the NHN only when they are so long that the point representation of an interchange will "seriously" misestimate travel distances. The most common use of explicit ramps are for the offset interchanges frequently used on turnpikes. Again, the design ideal for node location is the intersection of centerlines of the incident routes. Again, it is rarely achieved in directional interchanges, and instead is simply one of many overcrossings of ramps.
(c) Straight-line links. A link may have been added quickly, where a routing application required its existence, but it was deemed not cost- or time-effective to incorporate a shape from external data, such as TIGER. (A shape may be acquired at a later date in a more general upgrade of the state.)
(d) No source data exists. There are many new roads in the NHN which do not have representations in either the DLGs or TIGER, and many proposed routes whose eventual location is uncertain. Of necessity, their locations must be approximated from (usually) a small-scale map, at best through heads-up digitization on a TIGER background.
(e) Thinning. Most export versions of the NHN use at least 30 m thinning of the original DLG link shapes. This is about the magnitude of digitization noise, so this should have only a small effect on accuracy. Customized versions, however, have much more severe thinning to reduce file sizes, which removes a lot of "signal" (accuracy) as well as noise. [The standard public version currently uses 35 m thinning.]
The primary purpose of this network is to simulate vehicle routing. The topological structure of the network, or its "connectivity model," was chosen to do minimal damage to this objective, but is still a compromise among other objectives of geographic fidelity, simplicity, and low maintenance effort. Several aspects of this structural model may have an impact on applications using this network.
(a) Non-planar. A route which passes over another without any connection that traffic can use will be represented without a common node at the crossing.
(b) Explicit ramps. Most interchanges are represented by a single node at the intersection of centerlines. However, if this misrepresents the average distance of travel through the interchange by more than a half kilometer or so, the interchange will be decomposed into multiple links and nodes.
(c) Notional ramps. Especially in congested areas, it sometimes happens that a common set of ramps will serve to connect an expressway with several nearby surface streets. Rather than have separate nodes at the crossing of the expressway with each street, a single node along the expressway will represent the "interchange," and links will run from this node to surface street intersections which are not locationally displaced. These notional ramps will approximately overlay the expressway.
Ramps can be identified through a sign route that terminates in "RM" and an administrative class of "J" for an Interstate-related parent highway and "Q" for other highways.
Two files comprise the national network (or any particular extracted subnetwork): one describing links and the other nodes. Normally, .NDR is used as the file name extension for node files and .LKR for standard format link files. The following sections detail the attribute fields used in each file.
When the network is distributed in state subnetwork form each state will have its own node and link files. Each state's node file will therefore include all nodes internal to that state as well as nodes on the state boundary that it shares with a neighboring state. While state subnetworks have the advantage of relatively small files and make possible completely independent state applications, a national level application will require the unification of state node lists so that boundary nodes will not be duplicated.
The node file contains a single 60- to 72-byte logical record (depending on export version) for each node referenced in the link file as a link endpoint. Each record contains the 7-digit node number, the node's location, and optionally an alphabetic node name and other characteristics. The longitude and latitude of each node will be the same as the first or last digitized point in the location chain of every link that uses the node as an endpoint.
Node record fields and locations are:
(2) Node ID. As is true for link IDs, the first two digits of the node number are the FIPS code of that node's state. However, nodes on state and international boundaries begin with a prefix of '00'. No individual link will cross a state boundary, and most (but not all) boundary nodes will connect links in two separate states. All Canadian and Mexican boundary nodes have IDs of the form '0009xxx'. The low order five digits are the sequence number. Sequence numbers below 10000 are intended to be permanent (i.e., 'ss0xxxx'), but above that numbers should be considered temporary. Most of these IDs were transferred from an external source, usually UTTC digitization or imported State network files.
(5) Name. Roughly two-thirds of the nodes have been assigned names of nearby towns, geographic features, or freeway exit numbers. Node names are not necessarily unique. When names are used, the first two characters are the state postal abbreviation. Names may contain lower-case characters, apostrophes, or semi-colons.
flag. Added on an exception basis. Most of our local applications
software will assume that a junction is at-grade unless (a) there
are two or more controlled access links emanating from it, or (b)
the node has one of the following grade separation flags. [This flag
is expected to be expanded in the near future.]
source. A letter code. Nodes receive a code only when they are manually
edited. Nodes with positions derived from incident links are generally
(9) Update code. Same as for links.
(11) Facility code.
(12) Parent node. If the node is part of the expansion of an interchange, this field contains the 4-digit sequence number of the single master node in the same state that represents the interchange. If the node is a boundary node, this field contains the two FIPS codes of the states that adjoin the boundary.
file contains a variable number of contiguous records describing
each link. Each record is an 80-byte card image.
A typical FORTRAN code for recovering the digitized chain (where unit 41 is the link file) could be:
In this example XCOORD(1,8) would be the longitude of the 8th point in the chain and XCOORD(2,8) the latitude, in degrees and decimal fractions of a degree. Longitudes west of Greenwich are by convention negative. Eastern hemisphere locations in the state of Alaska also have longitudes measured in degrees west of Greenwich (i.e., less than -180). At the present time, no link's chain has more than 800 vertices, and no link in the public version has more than 500, but users are advised to include a test.
LINK HEADER RECORD FIELD NAMES AND LOCATIONS
The columns denoted as "bit fields" each contain a single hexadecimal digit representing four bits for four binary (yes/no) flags, with the bits numbered from left to right. For example, the digit "3" repre- sents 0011 [that is, 1-No, 2-No, 3-Yes, 4-Yes], and "D" represents 1101. A blank is the same as a zero. The user must decompose these codes to use them. In a low-level programming language the code can be read directly as a bit string; otherwise, you must read it as character data and enumerate each of the 17 possible values ("0" - "F" and blank) and their translations into the four binary variables it may include.
(1) LINK TYPE
blank - Standard
link for all applications.
The public version includes standard, A, and Z links, and excludes B, C, X, M, N, and D links, and the type field is cleared to all blanks.
(2) LINK ID
(2a) State prefix. The first 2 digits are the FIPS code of the state the link is contained in, from 01 to 56, and therefore provide a means of dividing the national network into subnetworks by state. In addition, 72 is used for Puerto Rico, 88 for Canada, and 91 for Mexico.
(2b) Link sequence number. Originally, within each state, the links as provided from the USGS file were simply numbered from 1 on, in the same order. New links which were not subdivisions were added to the bottom of the file, incrementing the sequence number by one. There will, however, be some small gaps in the sequence. Five digits are provided for the sequence number even though the largest sequence number is presently less than 9000, i.e., the leading digit of the sequence number is always 0. [This digit may be used for some other purpose in the future.]
(2c) Subdivision number. During the editing process when a parent link is subdivided into several daughter links, those daughters will retain the same sequence number, but will be assigned unique subdivision numbers from 1 to 9 in the last character position of the link ID. Links which have not been subdivided will have a zero in this position. Eventually, all subdivisions will be in order, but currently you cannot depend on this. Many parent links have been (or will be) divided into more than 9 daughters. When this happens two new "daughter" links with different sequence numbers (and therefore no backward reference) are established from the parent, and they are subdivided.
(3) SIGN ROUTE
Sign route types:
Source: Over the years link distances in version 1 of the NHPN had been refined through a combination of geographical calculation and transcription from maps and databases. Experience and several types of tests had led us to conclude that, on average, distance error was under 0.3% for important roads, even though individual links could easily be off by a mile. In other words, we had confi- dence in the global performance of the network, but not in facility-specific information.
The incorporation of the 100K DLG link shapes provided an opportunity to vastly improve individual link distance estimation, and even improve average network performance, by calculating link length as the sum of great circle distances (on an ellipsoid) of the digitized segments between vertices along the link polylines. The geographical imputation of distance, however, is vulnerable to several types of error due to the nature of the digital data, the centerline model of roadways, and various mishaps in the incorporation of DLG shapes. Our conclusion was that a blanket substitution of geographically imputed distances would slightly improve average performance and would substantially improve most individual link distance estimates, but it would seriously degrade estimates for a small number of links which had problems in their geographic representations. The number was still large enough to make us concerned about global performance. Instead, we elected to retain the existing distances pending a link-by-link examination for problems, and to substitute geographically imputed distances only after looking at the links. In addition, the discrepancies between original and imputed distances were a helpful clue for identifying possible shape prob- lems.
This process of improving length estimates is still continuing. In general, links which have an update code different than blank may be assumed to have either geographically imputed lengths that are reliable, or else something that is for some reason better. For other links with source codes different than blank or "J," users may at their option replace the given length with a geographically calcu- lated one, and the results should be somewhat "better" under most definitions. Note, however, that there is always a little ambiguity about what distance is -- whether it includes slopes, cutting corners, weaving, negotiating circuitous interchange ramps, etc.
Also, geographical imputation in earlier years did not take account of digitization noise. Although confidence is low because of the many countervailing and larger magnitude sources of error, the best estimate we can make is that link distances in the NHN remain a 0.2% overestimate of "true" planimetric distances.
(6) URBAN FLAG
(7) ONE-WAY INDICATOR
1 - Traffic flow permitted from A- to B-node only. Most parallel one-way streets are presently generalized as single two-way links. 2 - Bi-directional link. 3 - Traffic flow from B- to A-node only.
M - Divided highway (with median). C - Undivided (i.e., "centerline"). F - Ferry.
Source: State highway maps, supplemented by HPMS.
(10) ACCESS CONTROL
U - Uncontrolled access. G - Partially controlled access (with some at-grade intersections). I - Fully controlled access. All intersections are grade separated interchanges. F - Ferry.
Source: State highway maps, supplemented by USGS large scale maps and HPMS. There is often ambiguity about the definition of partially controlled access.
(11) NUMBER OF
Source: State highway maps, supplemented by HPMS. Most maps will not distinguish between 2-lane and multilane undivided highways, hence many roads can be expected to be misreported. In rare cases an actual value greater than 4 was reported by a state DMC or transcribed from HPMS, but for consistency it would be wise to treat all otherwise identical roads with 4 or more lanes the same way.
(12) TRAFFIC RESTRICTION
In order of priority:
Z - No vehicles (generally a passenger ferry). P - Closed to public use. C - Commercial traffic prohibited. H - Hazardous materials prohibited. T - Large trucks prohibited. Q - Occasionally closed to public. L - Local commercial traffic only. W - Normally closed in winter.
Completeness: Not systematic in any state; a blank field is no indication that a traffic restriction does not exist. This informa- tion has usually been added on an exception basis; that is, to solve a detected routing problem. Parkways and national parks were assumed to be restricted. Roads flagged in HPMS were also included.
(13) TOLL FLAG
T - Toll road. B - The link contains a toll bridge (or tunnel). F - Free passage. Roads not flagged can be assumed toll free, but ferries are uncertain unless explicitly marked.
(14-1) HM-164 route.
(15) DESIGNATED TRUCK ROUTEA - State designated route for STAA-dimensioned vehicles (the "Staggers act" National Network). When complete, all roads in the state subnetwork which have been designated will be flagged, but in many states there are additional designated roads not included in the network. B - Federally designated National Network routes for large commercial vehicles (a subset of State designated routes). C - While nominally a Federally designated route, there may be construction-related restrictions for an extended period.
Source: Federal Register (23 CFR 658), HPMS, and subjective assumptions when new parallel roads have been opened. All Interstate routes are assumed to be included unless Federally exempted (regardless of State intent).
(16) STRAHNET FLAG
A, a - STRAHNET, class 0. B, b - STRAHNET, class 1. C, c - STRAHNET, class 2. i - STRAHNET connector, priority 1. j - STRAHNET connector, priority 2. k - STRAHNET connector, priority 3. z - ORNL additions to maintain connectivity. 2-7 - Former STRAHNET routes. 8 - Former temporary STRAHNET routes to carry traffic when the permanent parallel route is not yet open.
Upper case is used for MTMC designated Oversize/Overweight routes; otherwise lower case flags are used.
(17-1) Major highways
subsystem. A "major intercity highways" subnetwork was
constructed by using all Interstate links, most (but not all) rural
principal arterials and urban expressways, and subjectively selected "gap
fillers" deemed to be logical extensions of rural principal
arterials (most of which are urban principal arterials). This subnetwork
depends on proposed routes (not yet open to traffic) for its connectivity.
Many of the included routes are parkways closed to commercial traffic,
particularly in New York.
(18) PAVEMENT TYPE
Source: State highway maps, for unpaved roads. The P/Q distinction considered in part average daily traffic and lane width from HPMS where available or an indication of a lower road class on the state highway map or USGS map. Occasionally the distinction was made to correct a routing a state DMC declared unrealistic. But there were no hard rules covering the choice.
(19) ADMINISTRATIVE CLASS
- Federal-Aid Interstate.
All FAI roads are marked. Note that the FAP, FAU, and FAS designations are mostly of historical interest after ISTEA passage. However, the FAI and FAP systems still form a connected subnetwork of important roads smaller than the entire NHN.
Administrative and functional classes are generally not indicated on a small number of short connecting links identified as ramps or local surface streets in the sign route field. Consequently, when constructing subnetworks based on these attributes, connectivity at some legitimate junctions may be lost unless tests for the existence of these connectors are made.
There are a number of cases where a portion of an existing open route is of a lower administrative or functional class because a new alignment is planned. Rather than explicitly add a new parallel projected link, the existing link may be flagged with the administrative and functional classes of its proposed replacement (except that surface streets will not be flagged as expressways).
(20) FUNCTIONAL CLASS
Functional class identifiers use the standard codes in HPMS, plus special codes with a leading blank (represented below as an underscore, "_") to indicate combinations where part of the link is inside and part outside an urbanized area boundary. As a rule of thumb, if less than 10 to 20 percent of a link's length AND less than 2 miles of a link is of a different functional class, a combination code will not be used. [The same rule applies to administrative class.] It should be noted that it is often difficult to be precise about the location of urbanized area boundaries without a current large-scale map that will show them. The typical situation is that boundary locations were estimated from milepoints in HPMS.
The functional class "05" is used in states which distinguish an intermediate rural arterial class, and in some other states which indicate minor arterials of different importance. These roads are reported as "06" in HPMS.
Some states have used codes of "13" and "15" in HPMS. When used, "13" was generally for urban extensions of rural principal ar terials, but both fell under the FHWA definition of urban principal arterial (class "14"). Starting in 1996, ORNL routinely distinguishes among urban principal arterials by using the "13", "14", and "15" codes to recognize subjective gradations of importance. Starting in 2002, ORNL distinguishes between rural expressways and surface roads by dividing FHWA class "02" into "02" "03."
Notes: Because of the special meaning of a leading blank, this field cannot be treated as a numeric variable without prior user modification. Because Interstate ramps often receive functional classes of "01" or "11," an administrative class of "I" should be used to identify Interstate highways rather than functional class.
Completeness: With only a handful of exceptions yet to be corrected, all rural minor arterials are included (relative to the NHPNv2). In urbanized areas, the completeness of principal arterials varies by state; consult appendix. In small urban areas, functional classes were basically guessed on the basis of instinct and pre-1993 status; completeness is unknown. Functional classes below "07" and "16" are rarely indicated because, even if reported in HPMS, their location and identity are usually impossible to estimate. There are very few roads in the network in these low functional classes anyway.
Source: HPMS, large
scale state or highway district level maps supplied by FHWA, NHPNv2.
This field contains
a letter code for the source of the link's coordinate chain. Sources
A character code
for the date of the last link update (either location or attribute).
Source codes are:
(25) SPECIAL SYSTEMS
(25-2) ILS. Roadways
designated as part of the FHWA/HEP Illustra- tive System.
- Open to traffic.
In general, both link and node sequence numbers will be pre- served between successive releases. That is, the same sequence number will never point to logically different network elements in different versions, and logically equivalent elements will usually have equivalent or related identifications. However, there will still be many circumstances where equivalencies will not be trans- parent. Establishing equivalencies between elements may be required by any users who modify or add attribute information to their own network copy and wish to transfer it to a new version. The problem will also arise when users make their own structural changes, and wish to transfer their data onto another user's network. It is recommended that these users follow the same Oak Ridge conventions listed below in their own modifications if they wish to maintain transferability, although they must also explicitly mark new element IDs they establish so they may be distinguished from the same IDs which may be added in a future Oak Ridge version.
Several different types of structural changes can be expected to occur, each with different considerations for maintaining compati- bility.
(1) Link subdivision. An existing link may be subdivided, usually in order to add a new roadway junction or interchange, but sometimes to add control points, traffic generators, or to indicate a change in roadway attributes. In most cases the resulting links will have the same link sequence number as the parent, but will differ in subdivision number. When the parent link itself had a non-zero subdivision number, one of the daughter links may retain the same number. [In fact, there is no intention that subdivision numbers will be retained between releases, even when the links are physically identical. Endpoint nodes of subdivisions must be examined to establish this equivalency.] In rare cases, this convention will be ignored because more than nine subdivisions would otherwise result. When this happens, a new link sequence number has to be assigned to one or more of the daughters, and equivalencies can be recognized only by following a sign route chain between the endpoint nodes of the group of parent links with the original sequence number. This is unusual enough that it is probably best done manually.
(2) Link combination. There are many nodes which serve no purpose, that is, which divide a portion of a route into two links with identical attributes, but which are not junctions themselves. These links may be combined into a single link, retaining the sequence number of one of the incident links, and eliminating the node, which is then deleted from the node list. Alterna- tively, that node may be moved to a new location along one of the incident links, in effect establishing a new dividing point between the same consecutive links along the route. The node is then available to be used as a junction for a completely new link intersecting the old route without the necessity of establishing new subdivisions. Topologically, the route's structure will not have changed, since it will have the same sequence of links and nodes, although physically the node may be several miles from its original location.
(3) Junction detail. A common structural enhancement is to resolve detail in a junction formerly represented by a single node. In this case a group of closely spaced nodes will be established. Only one of them will have the original node number; the others will have new numbers. To the extent possible, the new short links connecting these nodes will logically be subdivisions of the original incident links, but new subdivisions will always form a linear sequence along the original route. Therefore, there will often be some completely new link sequence numbers within the group. It also follows that the endpoint node of the original parent link sequence may be changed to another node within the junction group. [The ideal way to handle this problem would be to recognize daughter nodes, either by expli- citly noting a parent node number or by having node "subdivi- sion" numbers as is done for links. In some cases, this has been done by adding a "parent node" field to the node record, primarily to facilitate network collapsing, but it is the exception. This will probably become more common in the future as interchanges are treated in a more consistent and elaborate manner.]
(4) New alignment. When a new roadway is constructed to carry an existing route, the NHN may be updated in either of two ways. If the new route is close to the old (including reconstruction), or if the old route is removed or otherwise closed to traffic, the same link IDs will simply be transferred to the new align- ment. Logically, existing NHN links will be physically moved over the map surface with few if any topological changes. [Of course, links attributes like access control may be changed.] When this happens there should be no logical problems in conversion. Alternatively, if the old route remains a signi- ficant traffic artery it will remain in the network with its old ID, and a new link will be added and attached to the existing structure to carry the new facility.
Funding and assistance
for development of this network has been contributed by
We at Oak Ridge gratefully acknowledge the help we have received from these sponsors, and particularly the many individuals in National Guard state offices who have assisted in data acquisition and checking. Special thanks are especially due to innumerable research assistants who have painstakingly added and checked data over the years.