2.0 DATA SOURCES, AVAILABILITY, AND RISKS

2.1 Data Sources

As outlined in this section, the primary data sources for the traffic system test plan are the Mn/DOT Regional Transportation Management Center (RTMC) detection system, the Mn/DOT I-35W HOV lane quarterly reports, and the traffic count data from the City of Minneapolis. Data from the RTMC incident logs and Freeway Incidental Response Safety Team (FIRST) dispatch logs will also be used in the congestion analysis. These data are described in the safety test plan. Other special studies are also discussed in this section.

Mn/DOT RTMC Detection System Data. As part of the RTMC, Mn/DOT has deployed a system of traffic sensors to monitor traffic flow on the freeway system. These sensors are deployed in each freeway lane at approximately one-half mile intervals in both directions of travel. The sensors measure volume and loop occupancy at 30-second intervals. Mn/DOT stores and archives these data on a daily basis. These records contain the following data for each active traffic detector:

• volume – the number of vehicles passing the detector during a 30-second sampling period;
• occupancy – the percentage of time during a 30-second sampling period that the detector was occupied by a vehicle;
• flow rate – the total number of vehicles that would pass over the detector if the 30-second volume was sustained for a full hour (i.e., volume x 120); and
• speed – the average speed of all vehicles passing the detector during a 30-second sampling period. (Note: for Minnesota, speed is not a measured parameter but computed based upon the measured volume and occupancy.)

These sensor data will be used to derive travel time and throughput-based performance measures. The traffic sensors that will be used to partially derive the performance measures for the I-35W corridor, Highway 77/Cedar Avenue, and Highway 62 have been identified and documented in a separate file. Traffic sensor data will also be examined for the control corridors, which include I-394 and I-94N, and Highway 100, which represents a parallel route.

Travel time measures will be derived from the speed data while the volume data will be used to derive throughput measures. Speed values will also be used to assess whether or not a link is congested. It is envisioned that the same speed threshold (when the operating speed drops below 45 mph for any length of time) will be used to define whether or not a traffic lane is congested. The median value from the sensor data will be used to minimize the effects of outliers in the data. In developing the test plan, it is assumed that sensor data will be available for the general-purpose lanes as well as the HOT lanes and the PDSL.

These traffic sensor data could also be used to assess the effects of seasonal variations in travel and exogenous factors on traffic operations. By correlating traffic sensor data measures and exogenous factors (such as gasoline prices), it may be possible to determine the impact of these exogenous factors on traffic performance from a regional perspective.

Mn/DOT I-35W HOV Reports. The RTMC publishes quarterly reports on the I-35W HOV lanes and the I-394 HOT lanes. These reports include information on vehicle volumes, vehicle-occupancy levels, and vehicle classifications for the HOV and HOT lanes and the general-purpose freeway lanes. Information is also provided on the number of buses and bus riders, violation rates, and other related data. The quarterly reports have been prepared since the HOV lanes opened in the 1990s. As such, they provide both trend line information and pre-deployment information.

City of Minneapolis Traffic Count Database System (TCDS). The City of Minneapolis maintains a Transportation Data Management System (TDMS) which includes the TCDS. The TCDS includes data from traffic counts conducted every two years. The TCDS is available on the City of Minneapolis website and is a searchable database. Data from the TCDS will be used to assess impacts from the UPA projects on arterials in I-35W corridor within the City of Minneapolis.

Vehicle-Occupancy Counts. Vehicle-occupancy rates are needed for several of the Minnesota UPA evaluation analyses, especially those focusing on how the UPA projects influence the average occupancy rates. The vehicle-occupancy level is also a critical value in computing passenger throughput at both the facility and the corridor levels. Pre-deployment and post-deployment sampling of vehicle-occupancy rates for different classes of vehicles will need to be conducted.

Transit average vehicle-occupancy rates will be determined using transit passenger count data. Visual observance is still needed for other vehicles. For the HOT lanes and the PDSL, it will be important to count the number of SOV and HOV 2+ vehicles separately in the facility. In performing the vehicle-occupancy counts, data are needed on the number of vehicles in the following vehicle-occupancy classifications – SOV, HOV2, HOV3+, vanpools, and motorcycles. At a minimum, vehicle-occupancy rates should be sampled quarterly pre- and post- deployment of the UPA projects and just prior to the opening of a new deployment section. These counts could be conducted as part of the existing Mn/DOT quarterly reporting on I-394 and I-35W HOV and HOT lanes.

2.2 Data Availability

Most of the traffic system data discussed in the test plan is available from Mn/DOT. Traffic sensor and incident data are routinely archived and the data are made available to the public through the Mn/DOT website (http://www.dot.state.mn.us/tmc/trafficinfo/developers.html). Battelle team members have experience accessing and downloading these data from the Mn/DOT website.

As discussed above, special studies will need to be performed to obtain data that cannot be obtained through automated sources. These special studies include the average vehicle occupancy counts on the I-35W (both the general purpose lanes, the HOT lanes, and the PDSL).

It is expected that the special data collection effort will need to be performed quarterly and/or after significant changes in operating conditions occur in the corridor (e.g., after the opening of a major segment of a UPA improvement).

2.3 Potential Risks

The following summarizes some of the potential risks associated with the traffic system data for the Minnesota UPA national evaluation.

• Insufficient before data due to construction activities on I-35W. Currently, a significant portion of I-35W is under construction to implement UPA and other projects. Construction activities have resulted in the closing of some ramps, restricting travel to two lanes, and short duration closing of freeway segments. As a result, the pre-deployment data is not reflective of the before conditions. In addition, it appears that some detectors on I-35W in the evaluation corridor have not been providing freeway performance data due to construction activities. Data about current operations is needed in order to perform a good “before and after” evaluation of the improvements. This potential risk can be mitigated by examining historical data before major construction was initiated on I-35W.
• Gaps in data stream due to malfunctioning loop detectors or if instrumentation is not reinstalled and/or maintained. Procedures exist for filling-in for minor gaps in the traffic detector data; however, if gaps in the data stream are large due to malfunctioning loop detectors, or missing instrumentation, this could significantly impact the quality of the analyses. This potential risk can be mitigated by Mn/DOT closely monitoring the status of freeway detectors and repairing or replacing detectors that have failed or routinely provide erroneous data.
• Insufficient traffic performance data from arterial roadways. The City of Minneapolis TCDS will be used to examine the impacts of the UPA projects on arterials in the I-35W corridor within the City of Minneapolis. The ability to draw a meaningful conclusion from the TCDS may be limited.