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locations, sections/segments, and routes so that data entered particularly useful in development of safety plans if they can
or stored in one form can be linked to all other relevant data. be linked to the location reference system used in crash data.
The lack of intersection and interchange inventory data in
most highway agencies is a key limitation in safety planning
Traffic Volume Data
and safety management for intersections and for interchange
features such as ramp, speed-change lanes, and collector- State highway agencies collect and maintain data on traffic
distributor roads. It is hoped that the development of uni- volumes (Average Annual Daily Traffic [AADT]) for roads
form requirements for such inventory data (see the discus- on the state-controlled system. The AADTs are based on
sion of the Minimum Inventory of Roadway Elements counts made at both a limited number of permanent count
[MIRE] in Section XII of this guide) and the development of stations and a much greater number of sampling locations
tools like SafetyAnalyst that can use such data will encourage where 2- to 3-day counts are taken on each highway system.
the wider use of intersection and interchange inventory data. The standard is that the entire state system is covered on 2- to
Some local jurisdictions will also maintain inventory files, 3-year cycles. The "short" counts are then converted to
but many do not have them computerized or in a central AADTs using factors based on the day of the count, season,
location. Generally, they will be maintained and stored by and other factors, and are extrapolated to all sections of road-
different departments (e.g., traffic engineering, street mainte- way and to years when counts are not made at a given count
nance). The more extensive files will contain similar informa- station. The AADTs are either retained as a variable on the
tion to that collected by the states. Files on signalization at roadway inventory file, or in a separate file that is linkable
intersections are usually maintained for legal purposes, but are with the inventory. Not all states conduct counts on the rec-
sometimes not easily linkable with other inventory or crash ommended 2- to 3-year cycle. Even in states that do adhere to
data. Some localities also maintain supplemental files related the standard data collection cycle, traffic counts for some
to sidewalk presence, crosswalks, bicycle paths, bus stops, and scheduled locations are not collected. In such cases, states
other variables. typically replace the missing data with estimates.
In addition to AADTs, state agencies also collect and main-
tain large-truck counts or percentages for each roadway sec-
Highway Performance Monitoring System (HPMS)
tion. These are based on counts made with special equipment
Many of the above described state roadway inventory sys- that can separate vehicles into classes by length and number
tems were expansions of the HPMS system, a 1978 congres- of axles. These "classification counts" are usually made at
sionally mandated data system to collect data on the nation's many fewer locations than the basic traffic counts, so their
highways. HPMS is similar to the state inventory systems, but accuracy is less than for the AADT estimates. Supplemental
is based on a sample of locations from different functional truck counts may be made at other locations where "weight-
classifications in each state, rather than containing the full in-motion" systems are in place for use in truck-weight
state system. It contains limited data on all public roads. Data regulation efforts.
are inputted each year by each state, and collected, analyzed Local jurisdictions will also have traffic volume informa-
and reported to Congress by FHWA. While earlier versions of tion, but the consistency and quality varies by jurisdiction.
the system contained crash information for each sample While AADTs may be calculated for each city block in some
section, this is no longer the case. However, since HPMS cities, it is often the case that only intersection turning-volume
samples are usually flagged in the basic state inventory sys- counts made in signalization studies are available. In some lo-
tem, crashes could be linked with them. In general, the state cations, linkage of count data to other inventory files may be
analyst will use the state system rather than HPMS data in problematic. In general, traffic volume data is more limited in
state-based safety analyses. local jurisdictions than for the state-system roads.
Other Roadway and Intersection Driver History Files
Characteristic Data
Departments of motor vehicles maintain driver records of
Other data on roadway and intersection characteristics can all licensed drivers in the state. Driver records are typically
be obtained from aerial photographs. In particular, ortho- generated when a person enters the state licensing system to
photos are geographically converted to allow accurate meas- obtain a license or when unlicensed drivers have had a viola-
urements to be made. The ongoing development of asset tion or crash in the state. The record contains basic identifiers
management databases by state and local highway agencies (e.g., name, address, driver license number), demographic
will also provide a potentially valuable source of roadway and information on the driver (e.g., birth date, gender), and in-
intersection characteristics data. These data sources may be formation relevant to license and driver improvement actions
