Evaluating Alternative Operations Strategies to Improve Travel Time ...

SHRP 2 L11: Final Report
These data also serve as independent variables. When combined with traffic demand, they define
the causes of congestion and unreliable travel times. Therefore, to manage the roadway network
and to improve reliability, data that describe these disruptions as well as information on the
agency’s responses to these disruptions are required. Other SHRP2 research efforts and other
projects that explore techniques to operate the roadway system more efficiently will define more
detailed ways to quantify disruptions and the responses to those disruptions. For example,
projects SHRP2-L01 (2) SHRP2-L02 (3) and SHRP2-L13 (4) will all contribute knowledge toward
dealing with disruptions and the response programs to mitigate those disruptions. The following
are a “first cut” of these independent variables:
Traffic Incidents
The key incident variables are as follows:
• The number and type of incident (crash, disabled vehicle, fire, debris, abandoned vehicle,
injury severity if any, truck involvement, hazardous material spill) by time and location;
• A timeline for each incident (start of incident, detection, verification, on-scene arrival,
lane/shoulder open, all clear); and
• The number of lanes blocked and the duration of each blockage.
These variables describe the basic scope of the roadway disruption a given incident imparts. A
large number of additional variables are needed if the roadway agency expects to evaluate the
performance of any response that is sent to the scene. Both the National Cooperative Highway
Research Program and NTOC (5) have published comprehensive guidance on the selection and
use of these measures. NCHRP 20-07 – Task 215: Statewide Incident Reporting Systems (6) and
the NCHRP 20-07 – Task 202: Guide to Benchmarking Operational Performance Measures (7)
are two reports that provide more definitive advice on the measures that could be collected and
reported to manage and report on incident management systems. The intent, at a minimum, is to
track and report two measures:
• Roadway Clearance Time: the time between the first recordable awareness
(detection/notification/verification) of an incident by a responsible agency and first
confirmation that all lanes are available for traffic flow; and
• Incident Clearance Time: the time between the first recordable awareness and the time at
which the last responder has left the scene.
Weather
The key weather variables define the following:
• The existence and intensity (amount) of precipitation
• The existence and intensity (amount) of snow fall
• The existence of winds strong enough to disrupt traffic flow
• The existence of visibility restrictions
• The existence of temperatures low enough to cause ice formation, in combination with
dew point and precipitation information in order to predict (black) ice formation.
Strong consideration should be given to collecting these in the smallest time increment possible,
but not less than hourly. Data as geographically as precise as possible is most useful. That is,
weather data from a local airport are good, but the weather occurring on a roadway five miles
away from the airport can be considerably different, especially with regard to the timing and
GOALS AND PERFORMANCE TARGETS Page 29