Transportation Management Plan Effectiveness Framework and Pilot
Chapter 2. Useful Measures-of-Effectiveness for Evaluating Transportation Management Plan Strategies
Ways in which Transportation Management Plan Strategies Mitigate Work Zone Impacts
Not all of the available TMP strategies mitigate work zone impacts in the same way. In general terms, work zones can adversely impact safety and mobility by:
- Reducing available traffic-carrying capacity through the work zone, which can increase travel times and create traffic queues.
- Causing a redistribution (diversion) of travel to other routes, time periods, or travel modes.
- Creating unexpected driving conditions that surprise travelers and can increase crashes.
- Reducing available recovery areas for motorists who deviate from the designated travel path.
- Creating frustration and anxiety with the traveling public, residents, and businesses.
Consequently, TMP strategies attempt to mitigate impacts by:
- Increasing the amount of traffic-carrying capacity through the work zone or on alternative routes.
- Performing work that reduces capacity when traffic volumes are lower.
- Encouraging additional travel diversion away from the work zone beyond what would have happened otherwise.
- Reducing traveler surprise to unexpected conditions and features.
- Encouraging safer driving behavior through the work zone.
- Reducing the consequences of an errant vehicle leaving the travel lane relative to what would have happened if the mitigation strategy had not been employed.
- Reducing the consequences of crashes that do occur.
- Reducing other worker accident risks by providing more work space in which to operate.
- Reducing the duration of the work zone.
- Reducing public frustration and anxiety about the work zone.
Tables in Appendix A summarize how the different TMP strategies available attempt to mitigate work zone impacts. Some TMP strategies are highly-focused, attempting to address a single type of work zone impact. Other strategies, however, affect a broader range of work zone impacts. The tables also present subjective assessments as to the frequency with which the strategies are applied at work zones and the state-of-the-practice understanding of the effectiveness of the strategy in accomplishing these desired mitigation effects.
For many of the strategies that fall under the TTC plan categories, the effect of the strategy is on the available roadway capacity, either over an entire 24-hour period or during portions of the day when it is implemented. These strategies also affect the total number of capacity-reducing activities, or positively influence traveler awareness of the presence of capacity reducing activities, presumably also improving work zone safety. Assessment of the current level of understanding of the strategy effectiveness is qualitative, and all four dimensions of work zone effectiveness (mobility, safety, customer satisfaction, construction productivity and efficiency). For example, the effect of closing a travel lane for worker safety is fairly well understood in terms of how that strategy will affect road capacity. However, its effect in terms of reducing worker accident risk, or on how such strategies are perceived by the customers (likely, this depends on whether it creates excessive congestion when implemented) is relatively low. Thus, an overall assessment of medium is provided. Strategies that require or encourage drivers to find another route are assigned a low level of understanding, as abilities to predict such route choice changes and the subsequent effects on mobility and safety in and around the work zone as well as on other routes in the corridor is very limited. Similar assessments apply to understanding of lane shifts, shoulder closures, crossovers, reversible lanes, and detours/use of alternative routes. In all cases, strategies in the temporary traffic control category are considered and assessed at the project level.
The intended effect of most strategies included in the public information and outreach component categories is to change travel patterns (route, departure time, trip location, or mode choice) that collectively affect mobility, and/or to increase driver caution and thus safety. To date, little is actually understood as to how much these strategies accomplish this intent.
Finally, the intent of the demand management strategies within the transportation operations component categories is to reduce vehicular use during peak travel times and thus improve both work zone safety and mobility. In general terms, capabilities exist to estimate how such changes will affect operating conditions (mobility), and to a lesser extent safety, once the amount of vehicle use change is known. What is not known is the extent to which such changes to usage will actually occur. In contrast, the intent of the corridor/network management strategies is to increase vehicular throughput (capacity) of the various alternative roadways to better handle increased traffic demands due to demand management efforts or natural diversion by drivers. Again, the ability to predict how traffic demands will change and to which routes, is very limited. Work zone safety strategies are implemented to reduce vehicle crashes, crash severities, or worker accidents. Use is generally fairly high, but effect on safety is relatively unknown. Finally, traffic/incident management strategies are typically implemented to improve reaction to incidents that occur, incidents that because of capacity reductions in the work zone or because of a shift in vehicle demand on alternative routes have an even greater impact on mobility and safety. So long as demand volumes can be estimated, knowledge of how these strategies can reduce response times can be used to estimate how mobility can be improved.
Types of Transportation Management Plan Strategy Measures-of-Effectiveness Available for Assessing Effectiveness
Ideally, the MOEs used to evaluate the effectiveness of each TMP strategy should relate directly to these desired effects. For some of the strategies, the appropriate MOE to evaluate its effectiveness is fairly straightforward; for others, MOEs are much more challenging to identify and utilize. For example, measuring the amount of additional capacity that a particular strategy may provide at a work zone would be straightforward, but attempting to measure the amount by which a strategy may reduce traveler surprise to unexpected conditions would be more difficult. Further complicating matters is the fact that many of the strategies can have multiple impact mitigation effects. For instance, a work zone intelligent transportation system (ITS) deployment could potentially have a capacity-increasing effect, a traffic diversion-increasing effect, a safer driving effect, and a driver anxiety and frustration-reducing effect. Also, some of the mitigation strategies can have beneficial effects towards one or more of the types of work zone impacts listed above, while creating adverse effects upon others. An example of this situation are work hour restrictions that reduce the effects of project work activities upon commuter travel, but which may adversely affect the speed at which the agency and contractor can complete the project.
To best characterize different ways in which the various TMP strategies can influence work zone impacts, it makes sense to examine them within the context of previously-published FHWA guidance on work zone performance measures. (4) As part of that guidance, four dimensions of work zone impacts are identified:
- Customer satisfaction.
- Agency and contractor productivity and efficiency.
Mobility MOEs are further defined in terms of:
- Travel time reliability.
Similarly, safety MOEs are defined in terms of:
- Vehicle crashes.
- Operational surrogates of vehicle safety.
- Worker accidents.
These measures can then be further subdivided to reflect critical subsets of time (e.g., peak periods, when lane closures are present) as well as different facets of the feature (e.g., duration of queue presence, average delay per vehicle, crashes per million-vehicle-miles). The guidance document provides a number of suggested metrics (4), but there currently does not exist a nationally-agreed upon set of work zone MOEs that should be used for evaluating effectiveness.
In addition to mobility and safety impacts, many agencies are sensitive to how work zones affect public perceptions and opinions. Customer satisfaction MOEs are further defined in terms of:
- Work zone quality ratings (visual perceptions).
- Ratings of condition of travel through the work zone.
- Complaint frequency.
Finally, agency and contractor work productivity and efficiency represents the fourth dimension measures, and is most appropriate for evaluating strategies that reduce the duration of a work zone, or that phase of the work zone where safety and mobility impacts are most significant.
Agency/contractor productivity and efficiency measures are defined in terms of:
- Percent of allowable days worked.
- Percent of lane closure hours occurring outside of specially-allowed "work windows."
- Work productivity measures already tracked by an agency (e.g., tons of asphalt laid per day, cubic yards of concrete placed).
- Average hours of work during activities that adversely affect mobility and/or safety (i.e., lane closures).
- Average duration between road repairs on a facility.
Mapping all four these potential work zone MOE dimensions to each of the individual TMP strategies reveals a number of important considerations about how to best evaluate their effectiveness. First, for many of the strategies, one sees that their effect could be positive or negative, depending on the site conditions where it is implemented and how driver behavior is affected. As Table 4 illustrates, the effects of strategies that require detours or diversion, for example, may be positive for traffic safety at the work zone but increase crashes on alternative routes, possibly to the point that the net regional effect on safety is negative. Similarly, the effect of ramp metering on customer satisfaction will likely be positive for those travelers who experience better travel on the main lanes, but be negative for those who normally use the ramp. The table also illustrates that many of the strategies can have effects in some or even all of the four main dimensions of work zone safety and mobility performance. This means that, in many cases, the effects of individual TMP strategies that are implemented together as part of the overall TMP will often overlap and confound with one another and be more difficult to ascertain the relative contributions of a particular strategy upon the overall reduction in work zone impacts from what would have occurred otherwise.
Appendix B provides a series of tables that assess the MOEs likely to be influenced by the other TMP strategies available for use. Also included in these tables is a qualitative assessment of the relative costs of each of the potential TMP strategies available for use, and notes further explaining the effects expected (particularly when those effects are expected to be negative), and site conditions that would contribute to those effects. These tables should be useful to analysts when determining the type of assessments to be used in future TMP evaluations.
Accounting for Transportation Management Plan Strategy Interdependencies and Typical Deployment "Packages"
Although the confounding of effects of multiple TMP strategies does detract from efforts to assess the relative contribution of a particular strategy upon the overall success of a TMP, there are cases in which it does not make sense to attempt and isolate individual strategy effects. This is because many of the strategies themselves are highly interdependent. For example, the effectiveness of certain demand management strategies will depend on what other demand management strategies already exist or are being implemented for the project. Transit incentives might require improvements in transit service capacity in order to meet the anticipated increased demand, ridesharing/carpool incentives might likewise need to be coordinated with promotions of park-and-ride lots and programs, an overall public information and outreach campaign may be helpful to encourage travelers to shift to the newly-enhanced transit options. As another example, many of the traffic operations strategies can also be enhanced or coordinated with various types of public information strategies. In these situations, evaluating the overall effectiveness of the strategy "package" that was implemented would be the appropriate analysis approach.
|Full Roadway Closures||$$||Impacts of full closures on mobility and safety measures throughout corridor may be positive or negative, and would need to be measured against other traffic-handling options available. Strategy would be expected to improve worker safety.|
|Off-site detours/use of alternate routes||$$||Effects on safety depend on quality of detour/alternative route used.|
|Ramp metering||$$$||Reduction in vehicle demand could yield reduction in crashes, but could also increase those on other routes if diversion occurs. Customer satisfaction would be positive for main lane drivers, but negative for ramp users. PE effects would exist if mobility improvements assist materials and equipment delivery.|
M=Mobility, S=Safety, CS= Customer Satisfaction, PE=Agency or Contractor Productivity and Efficiency
Appendix C provides an assessment of the more common interdependencies between strategies that might exist within an overall TMP. The identification of interdependence does not automatically imply that both strategies must be deployed, but both typically are assessed together to determine whether actions pertaining to both need to be implemented in a coordinated manner. For example, one sees that full road closures and freeway-to-freeway interchange closures are interdependent with many of the TMP strategies listed, whereas off-site detours/alternative route use has interdependencies with many corridor/network management strategies. Demand management strategies tend to also be highly interdependent, providing opportunities to shift modes while also providing incentives (either monetary or travel time) to make such a shift. Demand management strategies are also highly interdependent upon several public information strategies and a few traffic/incident management strategies. It should also be noted that some of the strategies serve primarily a dependent role within a given TMP relative to the other strategies. This does not mean that they cannot be selected and implemented on their own to address a specific need within the overall objectives of the TMP, but their selection does not typically invoke considerations of other supporting strategies.Previous | Next