Recurring Traffic Bottlenecks: A Primer
Focus on Low-Cost Operational Improvements (Fourth Edition)
Chapter 7. Emerging Bottleneck Treatments
In addition to the innovative treatments discussed in Chapter 6 that have been used in practice, the recently completed Traffic Bottlenecks: Identification and Solutions study (Reference 13) developed three types of treatments that have not yet been implemented but that the authors found compelling in terms of benefits. These treatments are discussed below.
Dynamic Hard Shoulder Running
Dynamic Hard Shoulder Running (HSR) is an extension of typical HSR, but instead of the shoulder being open to traffic on a fixed schedule, it is only opened when congestion is present at a pre-defined level. The authors discuss dynamic HSR in the context of nonrecurring congestion (e.g., incidents) but in fact it could apply to congestion caused by a fixed bottleneck as well. The advantage of dynamic HSR is that the shoulder remains open for safety reasons when it is not needed for dealing with congestion.
Simulation analysis of dynamic HSR revealed the following findings:
- Dynamic HSR strategies are more suitable for property damage only (PDO) incidents where traffic management centers have more flexibility in managing traffic.
- Only the part of the shoulder that is 0.5 mi upstream and downstream of an incident location needs to be opened to fully use the potential of shoulders for incident management.
- The opened shoulder section can be closed as soon as possible after the incident is cleared. Opening the shoulder for a longer time will not improve traffic flow conditions.
- The effectiveness of dynamic HSR is rather significant across different roadway geometry, traffic, and incident scenarios. 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, which are very significant considering only opening a certain section of the one-lane shoulder within a limited amount of time.
Dynamic Reversible Left Turn Lanes
The diamond interchange with its signal control has been successfully used in numerous freeway-to-arterial connections. However, the operation of a conventional signalized diamond interchange design is now becoming a challenging issue, particularly in an urban or suburban environment where heavy traffic volumes must be served and right-of-way is restrictive. By removing the internal center back-to-back left turn lanes and creating reversible lanes, the signal timings can be set to give each left turn movement use of the full-length lane during each of the opposing left turn movement.
Operational benefits of the dynamic reversible left turn (DRLT) lane design, as analyzed with simulation and relative to the conventional diamond design, were as follows:
- The simulations showed increased left turn on-ramp movement throughput, reduced network travel time, and reduced delay for various DRLT lane designs. The effects were most prominent when the number of through lanes was proportionally higher than the number of left turn lanes.
- DRLT design is not applicable for all diamond interchange scenarios, but for those with high volumes and a low number of left turn lanes. The best implementation of the new interchange design may be during certain times of day where the left turn on-ramp volumes are high. Throughout the day when the turning movements are balanced, the intersection can remain a conventional three-phase design. However, when the movements change (possibly during afternoon peak) the DRLT design can be beneficial and implementable.
Contraflow Left Turn Pockets
During the peak hours of traffic congestion, some signalized intersections suffer from excessive queue spillover of left-turning vehicles into adjacent through lanes. The innovative contraflow left turn (CLT) pocket treatment aims to mitigate the problem by dynamically allocating lanes in the opposing direction to create an additional left turn pocket lane. By adding additional capacity to the left turn movement, delays for the entire intersection can be reduced, but note that the intersection exit at the bottom is reduced to a single lane. This feature may add additional delay to the downward through movement as shown in the figure if demand for the through movement is high enough. In some cases, the additional left turn capacity could allow left turn green times to be reduced such that green time could be reallocated towards other turning movements at the signal.
Although this design treatment requires pre-signals to control entrances to the contraflow turn pockets, it would not require advanced vehicles or infrastructure. As a result, it has the potential of being a cost-effective bottleneck mitigation strategy.
Simulation results showed that the CLT treatment produced the most delay savings under high travel demands. Another finding was that the highest percentage delay reductions occurred when northbound and southbound turning movement demands were balanced (i.e., almost equal). Total intersection delay was reduced between 7 and 22 percent under various scenarios studied. Therefore, the case study simulations indicated that CLT pockets are capable of significant overall delay reduction if the right demand and signal geometry conditions exist.