Work zone impact thresholds
The Oregon DOT (ODOT) has recently begun to emphasize the importance of mobility during construction. New procedures have been created to help manage mobility on a statewide basis, including the development of thresholds for work zone impact that are based on delay time. ODOT has defined work zone delay as the additional travel time that will be required to travel from one point to another as a result of construction activities. With this definition, recurring delay that results from existing capacity or geometric deficiencies is excluded. Similarly, non-recurring delay that results from incidents is also excluded.
Thresholds have been developed that are not project-specific, but were developed for roadway corridors, such as I-84 from Portland to the Idaho state line. Each corridor is broken into multiple segments, generally between 30 and 60 miles long. The corridor thresholds are composed of segment thresholds. Project-level thresholds are not used. Delay management takes place at the segment and corridor levels. Segment thresholds will be monitored though a given segment that may have multiple projects or groups of projects that may be active at any given time.
The method by which the thresholds were calculated takes into account varying location characteristics. The delay thresholds were established using the following methodology:
1. Using segment lengths and assumed travel speeds for off-peak travel conditions, off-peak travel times were estimated for each segment.
2. Travel times for peak travel conditions were then estimated by increasing the off-peak travel times. Off-peak travel times were increased by 45 percent for urban areas, 30 percent for semi-urban areas, and 10 percent for all other areas.
3. Delay thresholds were then calculated to be 10 percent of the peak travel times.
The thresholds that have been calculated to this point have been based on calculated base travel times. As time progresses, the calculated base travel times will be replaced by measured base travel times.
ODOT and the Oregon Bridge Delivery Partners (OBDP) are committed to meeting the established delay thresholds. However, it is recognized that specific work activities and time periods may make it infeasible to achieve the delay thresholds for a particular segment or corridor. For example, upon evaluation of a construction staging event it can be concluded that a lesser impact to the traveling public will occur by conducting short-term lane restrictions (a few days) in lieu of longer-term off-peak lane restrictions that cause minor delays over an extended period of time (several weeks), then it may be appropriate to establish specific days (and times of day) to allow the closures and resultant delays which exceed the delay thresholds. As another example, the additional cost of meeting the delay threshold through changes to the proposed project schedule, staging and/or contracting method may be prohibitive.
In these cases, an exception may be sought. The basis for exceptions would be an inclusive evaluation of the following minimum criteria: 1) Alternative analysis demonstrating the least amount of overall delay impact and potential economic impact to communities and businesses; 2) Direct project cost comparisons of each staging alternative; 3) Ability to communicate with the traveling public and trucking industry to gain buy-in and awareness of the impacts and means to mitigate those impacts.
If an exception is necessary, it should be considered and processed during the planning and design stages of a project and not immediately before a project is to begin. Similarly, an exception will not be an acceptable means of addressing actual delay during construction.
Any and all exceptions will require approval of the ODOT Statewide Traffic Mobility Manager with concurrence by the appropriate Region Manager and follow-up review by the Statewide Mobility Steering Committee. It should be expected that requested exceptions will receive heavy scrutiny and will be approved only for unique and exceptional circumstances.
Upon approval of the exception, the delay thresholds will be modified to reflect the parameters of the exception. Such modifications will remain in effect for the duration of the specific project, after which the delay thresholds will revert to those documented in the Corridor-Level Traffic Management Plan (TMP).
An overview of the process of enforcing these thresholds in the field is as follows:
– ODOT sets delay thresholds for corridors & segments.
– Design Consultants recommend initial work zone traffic control.
– ODOT predicts delay impacts.
– Design Consultants modify work zone traffic control or the project schedule at ODOT’s direction to meet delay thresholds.
– Thresholds are NOT part of construction contract. The contractor builds per the plans.
– ODOT measures delay during construction.
– ODOT works with the contractor to mitigate delay when problems arise.
During construction, delays for each work zone, or group of work zones, will be measured and compared with the predicted delays. If the measured delays exceed the predicted delays, the cause of the inconsistency must be identified by the contractor and ODOT or OBDP mobility staff.
Work zones that are not operating as well as expected will be studied to determine possible causal factors. Work zone capacity per lane will be monitored and compared to the predicted capacity per lane to help improve future predictive techniques and to help identify specific traffic control plan (TCP) elements that may be causing a greater than predicted impact on traffic. A program-wide search will be undertaken for common factors in each of the poorly performing work zones to identify staging types or TCP elements that should be avoided.
Within the Corridor-Level Traffic Management Plan (TMP), a delay threshold is described for the corridor and for each segment within the corridor. The aggregated project and bundle delays will be summed for each segment along a corridor and for the corridor in its entirety. The observed segment and corridor delay will be compared to the projected segment and corridor delays (based on OBDP’s planning level analysis), as well as the segment and corridor delay threshold. (See the web site below for Delay Definitions Memo.)
Travel time runs along the corridor will be compared to baseline travel time runs (collected by OBDP prior to commencement of construction activities, or supplemented by calculations based on travel speed) to determine the amount of additional travel time required to travel the length of the corridor as a result of construction activities. In conjunction with the aggregated project and bundle delays, the travel delays resulting from STIP projects will be identified. The additional travel delays, which will include STIP, local, and OTIA III projects will be compared to the delay thresholds set forth in the Corridor-Level TMPs to identify problem areas. Coordination between the ODOT Regions and OBDP may be required to restore corridor mobility to its target efficiency.
In October 2008, the Oregon Work Zone Traffic Analysis tool went live online. The tool allows an analyst to determine appropriate windows for lane closures, shoulder closures, or controlled delays (rolling stops) on any roadway under ODOT jurisdiction. The tool also estimates delays that should be expected based on user-defined staging.
The tool accesses traffic data from a number of ODOT databases and also incorporates truck route information and geometric design information to be a single source for traffic, freight, and geometric data for traffic control plans design. The Oregon Work Zone Traffic Analysis tool can be accessed on the web site at http://wzta.obdp.org.