By: Dr. Yimin Tang and Dr. Steven I-Jy Chien
Integrating a travel time-based traffic diversion plan with a cost-effective work schedule can substantially reduce both costs and congestion.
Traffic congestion cost drivers $87.2 billion in the U.S. in 2007, according to the 2009 “Urban Mobility Report.” Drivers paid that bill by enduring 4.2 billion hours of time wasted during traffic delays and purchasing an extra 2.8 billion gallons of fuel to burn during traffic slowdowns or trip-lengthening detours. Work zones on freeways caused nearly one-fourth of the nonrecurring delays and one-tenth of overall delay.
Transportation agencies can use two strategies to reduce congestion in work zones. Careful scheduling is one approach. The number of workers and the amount of equipment might be increased in order to accelerate the work and reduce the project’s duration, or maintenance activities might be performed only during nighttime and off-peak periods. However, those kinds of adjustments increase the project’s cost and may increase safety risks for workers. The other strategy is traffic diversion. Shifting some of the traffic to diversion routes relieves the congestion through the work zone. But if it is not done appropriately, this approach may significantly degrade the level of service on the diversion routes.
Recent research shows that a coordinated effort to optimize the work-zone schedule and develop a plan for time-varying traffic diversion can dramatically reduce both project costs and congestion. The magnitude of these reductions highlights the importance of thorough planning. An agency can realize substantial savings by comparing various scenarios of work-zone activity rather than simply relying on usual practices. Devising effective time periods during which traffic diversion is encouraged can amplify those savings.
Crews and avenues
A hypothetical example demonstrates the value of integrating schedule planning with travel time-based traffic diversion. The example project is a 5-km-long pavement resurfacing project on a New Jersey freeway. It is located within a 7-km-long stretch between an exit ramp and an entrance ramp providing access to a 7-km-long minor arterial that parallels the freeway. The freeway has two travel lanes in each direction, and the diversion route has one lane in each direction. During the resurfacing project, one freeway lane will be closed at a time. Intelligent transportation systems (ITS)—such as an automated work-zone information system (AWIS) and a 5-1-1 traveler information system—alert drivers whenever using the diversion route would save them time.
The researchers considered four maintenance crew options, ranging from a low-cost, low-productivity crew (Crew 1) to an expensive, highly skilled crew capable of using an accelerated construction method and work schedule (Crew 4). The researchers first used computer simulation to find the optimal work schedule for a typical, or baseline, work crew (Crew 2) operating without traffic diversion. The unit cost and production rate values for this baseline crew were taken from the RS Means Heavy Construction Cost Data manual. For Crew 2, the optimal schedule consisted of five work-zone operations during three overnight off-peak periods and two midday off-peak periods. Separating those operational periods were four peak-period work breaks ranging in length from 2.75 hours to 5.25 hours. Based on this schedule, the total project cost would be $150,258 per lane. This total includes the costs of material, equipment, labor, setting up and removing the work zone between breaks, and idle equipment and workers during work breaks. It also reflects road-user costs of delay time, vehicle operation and work-zone traffic accidents.
The researchers then introduced the idea of diversion strategies based on traffic volumes that varied throughout the day and recalculated an optimal work schedule. In this case, the freeway’s average annual daily traffic (AADT) was assumed to be 45,000 vehicles per day (vpd), and the AADT on the diversion route was 25,000 vpd. Hourly traffic volumes were used to trigger periods of partial traffic diversion whenever the travel times on the mainline and the diversion route would be equal. When the projected travel time on the freeway was less than the time on the diversion route, no diversion would be necessary. However, when the diversion route was quicker, motorists would be advised to consider traffic diversion. Allowing for traffic diversion improved the optimal work schedule, reducing the number of work periods from five to two and reducing the number of work breaks from four to one. Under this scenario, the project could be completed in a 12-hour overnight work period followed by a three-hour work break during the next morning peak period, with a final work period lasting from 10 a.m. until 5:30 the following morning. Motorists would be advised to consider the diversion route from shortly after noon until 7 p.m. within the final work period. According to the simulation, 16% of the mainline traffic used the diversion route during the active diversion period. Compared with the prior analysis, which did not consider diversion, the per-lane project cost decreased by $8,693 (a 5.8% reduction), and the project duration decreased from 53.5 hours to 34.5 hours (a 36% reduction).
The researchers found an interesting pattern arising from the diversion strategy. During this seven-hour diversion period, the traffic flow on the freeway oscillated around the work-zone capacity. When the freeway became congested, the diversion route became more desirable and some of the traffic diverted from the work zone. This relieved the mainline congestion, reducing the need for drivers to avoid the work zone and, consequently, lowering the diversion rate. The diverted flow fluctuated over time because of the variation in travel times on the mainline and diversion routes.
Three’s cost efficiency
Seeing that traffic diversion improved the project’s efficiency for the baseline crew, the researchers next evaluated its effects when different compositions of work crews and construction methods were used. First, assuming no traffic diversion, they computed an optimal work schedule for each of the four hypothetical crews. The project durations ranged from 55.25 hours per lane (for the low-cost, low-productivity Crew 1) to only 34 hours (for the highest-cost, highest-productivity Crew 4). The project’s total per-lane cost ranged from $157,365 down to $143,836, with Crew 3 producing the least cost and a relatively short duration of 36.5 hours per lane. Using Crew 3 reduced the project cost by 8.6% compared with the highest cost option (Crew 1). The wide range of results underscores the importance of selecting a cost-efficient crew and construction method when diverting traffic is not feasible.
Repeating the optimization process with the addition of time-optimized traffic diversion strategies, the researchers obtained per-lane project costs ranging from $146,078 down to $141,273. The per-lane project durations ranged from 37.75 hours to 32.5 hours. Again, Crew 3 proved to offer the lowest cost option, but using the optimal Crew 3 reduced the project’s cost by only 3.3% compared with the most expensive option (Crew 1). Compared with the prior scenario, promoting traffic diversion narrowed the cost range considerably, decreasing the project cost by $2,563 per lane and reducing the project duration by an hour per lane. In other words, the crew choices may be less critical in this case when traffic diversion is properly planned.
Speaking volumes
The researchers then extended the analysis to evaluate the magnitude of potential cost savings and determine the traffic conditions that would drive the decision to divert traffic. Traffic diversion was considered across a range of freeway traffic volumes, from 30,000 to 60,000 vpd.
Figure 1 shows how the total cost saving from traffic diversion fluctuates as traffic volume increases in the example project. Although the curves would be somewhat different under different project circumstances, there appears to be a clear trend. The curves for Crews 1 and 2 indicate that the cost saving increases quickly for higher mainline traffic volumes. This suggests that diversion would be important if a less productive construction method and maintenance crew were to be chosen for a project. For Crews 1 and 2, the total cost with and without traffic diversion would be at the break-even point when traffic volumes are less than 35,000 vph; diverting traffic would be an economical strategy when the freeway’s AADT exceeds 35,000 vpd.
The two lower curves in Figure 1 indicate the diversion does not generate substantial cost savings for high-productivity crews and construction methods (Crews 3 and 4 in the example), even at high traffic volumes. The analysis yielded higher break-even points for Crews 3 and 4; their higher productivity rates would accelerate project completion and lower costs accordingly. In these cases, diverting traffic would be an economical strategy for Crew 3 if the mainline AADT exceeded 42,500 vpd, and for Crew 4 if the AADT exceeded 52,500 vpd. The higher break-even points for diverting traffic suggest that accelerated work methods and highly productive crews would be best able to handle the higher traffic volume scenarios when no diversion routes are available.
Look it all over
The hypothetical example described above shows relative cost savings and delay reductions for a specific project. Actual amounts will differ from one project to another. However, the analysis suggests some general conclusions that apply to all projects.
In general, the results suggest that diverting traffic would be beneficial if less productive maintenance crews are employed for work zones along heavily traveled highways. However, a more productive maintenance crew would be cost-effective if traffic were not able to divert to other routes due to roadway geometrics or congestion constraints.
In this example, the diversion strategy was governed by an attempt to balance the travel times on the freeway and the diversion route. ITS systems could be used to promote the dynamic traffic diversion pattern, encouraging motorists to take a quicker route to avoid traffic congestion and delays. The analysis showed that the best diversion strategy is to reduce the freeway traffic to the work zone’s capacity.
In addition to producing these general conclusions, the example described above underscores the importance of individualized planning for maintenance projects. A transportation agency should evaluate and compare several alternatives to work-zone activity schedules, crew compositions and construction methods to find one that minimizes the project’s total cost and traffic delays to motorists. Strategies for applying travel time-based traffic diversion should be part of that evaluation process. Even in the case where a work-zone schedule is predetermined by past practices or other considerations, devising an effective, time-sensitive diversion strategy can constrain the project’s financial and logistical impacts if one or more diversion routes are available as an option.
About The Author: Tang is a senior traffic engineer in the Piscataway, N.J., office of AECOM USA Inc. He can be reached at [email protected]. Chien is a professor in the Department of Civil and Environmental Engineering at the New Jersey Institute of Technology in Newar
