Recurring Traffic Bottlenecks: A Primer
Focus on Low-Cost Operational Improvements
Localized Bottleneck Reduction Strategies
Types of LBR Treatments
The following is a sampling of short-term, low-cost operational and geometric improvements. All of these remedies address operational deficiencies, as opposed to other congestion mitigation efforts that address driver choice, travel demand, corridor-wide upgrades, or simply (but expensively) building our way out of congestion.
- Shoulder conversions. The FHWA is currently studying the efficacy and prudence of using improved roadway shoulders to address congestion in particularly challenging situations. The safety implications of using shoulders, versus the congestion relief tradeoff of same, is first-and-foremost at the discussion of this strategy. This involves using a short section of traffic-bearing shoulder as an additional travel lane. Shoulder conversions are appropriate between interchanges or to provide lane congruency with adjacent sections. The improved shoulder should be rated for use as a travel lane. Practical challenges exist as to designing controls for part-time use versus 24/7 use.
- Restriping existing pavement in merge or diverge areas to provide additional lanes or to improve lane balance, provide an acceleration/deceleration lane, extend the merge/diverge area, or improve geometrics to better serve demand.
- Minor interchange modifications. Adding a new auxiliary lane to connect closely spaced interchanges, extending the length of an exit lane to store queues from a ramp terminus, and providing exit-only or “slip ramps” in advance of a major interchange are three examples. Note – major interchange modifications (e.g., an entire interchange rebuild) would tend to be outside the purview of the “localized” solutions found in this Primer.
- Lane width reductions. This involves reducing lane widths and restriping to add an additional travel and/or auxiliary lane.
- Modify weaving areas by adding collector/distributor or through lanes.
- Ramp modifications. These could include ramp metering; widening, extending, closing, or consolidating ramps; or reversing entrance and exit ramps to improve operations.
- Speed harmonization (variable speed limits). This is the practice of adjusting speed limits when congestion thresholds have been exceeded and congestion and queue forming is imminent. Speed harmonization can also be used to promote safer driving during inclement weather conditions. This mostly European practice reduces the traffic “shock wave” that results through congested corridors, thereby delaying the onset of a breakdown in traffic conditions. The result is decreased headways and more uniform driver behavior, which indirectly benefit bottlenecks and chokepoints.
- Zippering or self-metering that promotes fair and smooth merges. A motorist who is 10th in line knows that he will be 20th to merge into the single lane ahead. This helps to eliminate line jumpers that bull ahead, disrupt the queues, and often block adjacent lanes until they force their way in line. Usually this method of merging requires on-site enforcement, but often is exhibited by regulars who know the process and are willing to abide.
- Improve traffic signal timing on arterials. Also, traffic signal timing improvements at ramp terminal intersections will prevent ramp queues from backing up onto freeway main lanes.
- Access management principles to reduce vehicular conflicts (hence, delays) on arterial corridors
- Roundabouts. Roundabouts may be used in place of stop sign or signal controlled intersections, including replacing signalized intersections at ramp termini.
- Innovative intersection and intersection designs. A variety of new designs are being implemented around the country (see below).
- High-Occupancy Vehicle (HOV) or reversible lanes.
- Provide traveler information on traffic diversions.
- Implement congestion pricing. Congestion pricing entails charging fees or tolls for road use that vary by level of vehicle demand on the facility. The objective is to bring supply and demand into alignment.
In the past several years, several nontraditional designs have been developed for signalized intersections and interchanges. The alternative designs for intersections all attempt to remove one or more of the conventional left-turn movements from the major intersection. By removing one or more of the critical conflicting traffic maneuvers from the major intersection, fewer signal phases are required for signal operation. This can result in shorter signal cycle lengths, shorter delays, and higher capacities compared to conventional intersections. Exhibits 8 and 9 show examples for two of these innovative designs.
Exhibit 8. Vehicular Movements at a Continuous Flow Intersection
One such intersection design is the Continuous Flow Intersection, which eliminates one or more left-turn conflicts at a main intersection. This is achieved through dedicated left-turn bays located several hundred feet prior to the main intersection, which allow left-turning vehicles to move at the same time as through traffic. The left-turn traffic signal phase is eliminated, allowing more vehicles to move through the main intersection and thus reducing traffic congestion and delays. These at-grade intersections achieve traffic flow similar to grade-separated interchanges, but at a considerably lower cost. Other innovative intersection designs include:
- Displaced left-turn (DLT) intersection;
- Median U-turn (MUT) intersection;
- Restricted crossing U-turn (RCUT) intersection;and
- Quadrant roadway (QR) intersection.
Exhibit 9. Crossover Movement in a DCD Interchange
The double crossover diamond (DCD) interchange, also known as a diverging diamond interchange (DDI), is a new interchange design that has much in common with the design of a conventional diamond interchange. The main difference between a DCD interchange and a conventional diamond interchange is in the way left and through movements navigate between the cross street intersections with ramp. The DCD design accommodates left-turning movements onto arterials and limited access highways while eliminating the need for a left-turn signal phase at signalized ramp terminal intersections. On the cross street, the traffic moves to the left side of the roadway between the signalized ramp intersections. This allows drivers of vehicles on the cross street who want to turn left onto the ramps the chance to continue to the ramps without conflicting with opposing through traffic and without stopping.
