Bottlenecks are major causes of traffic congestion, and the mitigation or elimination of bottlenecks is believed to be a top priority. This report focuses on solutions involving dynamic lane use, contraflow or reversible lane use, hard shoulder lane use, lane width reduction, and the modest extension of auxiliary lanes. In addition to introducing a new approach for ranking traffic bottlenecks and creating a new playbook of 70 bottleneck mitigation strategies, this study presented a benefit-to-cost (B/C) analysis of five cost-effective bottleneck mitigation strategies and three innovative bottleneck mitigation strategies. A data-driven congestion and bottleneck identification software tool was created for ranking traffic bottlenecks, using numerous performance measures such as the duration, intensity, variability and extent of bottlenecks.
Bottleneck solutions in this report are divided into three categories. First, there is a playbook of 70 bottleneck solutions, which are further divided into seven categories classified by their geometry or operational challenges. Second, a set of cost-effective bottleneck mitigation strategies, currently underdeployed in the United States, were considered. Third, a set of low-cost innovative solutions--including dynamic HSR, dynamic reversible left turn (DRLT) lanes, and contraflow left turn (CLT) pockets--that are not yet deployed in the United States were discussed. The report focused on quantifying the B/C ratios of the low-cost bottleneck mitigation solutions. These are (i) dynamic lane grouping (DLG), (ii) dynamic merge control (DMC), (iii) changing acceleration lane lengths, (iv) hard shoulder running (HSR) and (v) reducing lane and shoulder widths to add new lanes.
When performing the B/C analyses, value of time was assumed to be 50 percent of the prevailing wage rate. This study used a national wage rate of $21 per hour in 2009. Assuming an average occupancy of 1.6 persons per vehicle, hourly savings was taken as $17 per vehicle. The benefits of each proposed bottleneck mitigation solution (e.g., savings in vehicle delay) were estimated using traffic simulation software tools. Regarding the cost of installing and maintaining a DMS, the unit cost entries for DMS from the U.S. Department of Transportation (USDOT) Knowledge Resources Web site was used. The B/C ratios estimated in this study were mobility-based B/C ratios not accounting for safety and environmental impacts.
- For DLG, case studies using microsimulation estimated a reduction of total intersection delay by 69.3 vehicle-hours during a one-hour simulation for a high-volume intersection (e.g., 6,800 vehicles/h) and 15.9 vehicle-hours for a low-volume intersection (e.g., 4,100 vehicles/h). Using the vehicle-hour savings from these two case studies, annual benefits in the range of $68,000 (low-volume intersection) to $295,000 (high-volume intersection) were estimated. The estimated B/C ratio was 7:1 for low-volume intersections and 29:1 for high-volume intersections.
- For DMC, benefits were sensitive to the combination of merging demand volumes and number of merging lanes. Simulated best-case scenarios estimated that three-lane merge with two-lane freeway with lane closure at ramp, and lane closure at mainline saved 100 seconds and 124 seconds per vehicle, respectively. Two-lane merge with two-lane freeway with lane closure at ramp and lane closure at mainline saved 202 seconds and 193 seconds per vehicle, respectively. Estimated benefits varied between $236,000 and $1,002,000 per year. B/C ratio varied between 10:1 to 44:1.
- Simulation results showed that increasing acceleration lane length from 500 to 1,000 ft and 1,500 ft would save at least 6.58 hours and 10.58 hours per day for the three-lane 40 mi/h configuration, respectively. Estimated annual benefits of increasing the acceleration lane length from 500 ft to 1,000 ft varied between $27,965 and $42,712; and benefits of increasing from 500 ft. to 1,500 ft. varied between $44,965 and $78,838. The B/C ratio varied from 1.3:1 (worst scenario) and 28:1 (best scenario).
- Multiple HSR scenarios analyzed in the traffic simulation software resulted in annual benefits varying between $103,200 and $418,200. The B/C ratio varied from 1.7:1 (worst scenario) and 29:1 (best scenario).
- Simulation analyses were used to estimate the benefits of reducing exiting lane and shoulder widths to increase the number of lanes. Increasing from four to five lanes (with narrower lane widths) would save 39,899 min (i.e., 665 h) during the three-hour peak travel period, and the annual benefit was estimated as $2.83 million. The B/C ratio was estimated at 30:1.
- For dynamic HSR, depending on the traffic condition and number of lane blockage, the average delay can improve by 30 to 80 percent and total traffic throughput by 15 to 40 percent.
- For DRLT, one case study showed that the less balanced the turning movements (higher number of left turns) the better the DRLT design performs. Benefits ranged from none at low left turn movements to 29 percent throughput improvements at high volumes and high left turn rates. The other case study had significant traffic flow improvements as measured by delays, speeds, and vehicle stops (ranging from 40.7 to 249.4 percent).
- Simulated results showed that the estimated delay reductions for CLT pockets were between 7 and 22 percent.