How Can Bottlenecks be Fixed?

Short-Term, Low-Cost Improvements

Here is a sampling of operational remediations.

1. Using a short section of shoulder as an additional travel lane.
2. Re-striping merge or diverge areas to better serve demand.
3. Reducing lane widths to add a travel and/or auxiliary lane
   (e.g., re-striping).
4. Modifying weaving (e.g., adding collector/distributor or through lanes).
5. Metering or closing entrance ramps.

Table 3. Mapping Bottleneck Problems to Mitigation Measures

Bottleneck Types	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures	Mitigation Measures
Bottleneck Types	Auxiliary Lanes	Collector-Distributor Road	Paved Right Shoulder	Paved Left Shoulder	Shoulder/Plus Lane	Re-Stripping to Add More Narrow Lanes	All Purpose Lane (Concurrent or Reversible)	HOV Lanes (Concurrent or Reversible)	Truck Reversible	Ramp Metering	Temporary Ramp Closures	Traffic Diversion Information
Heavy On-Ramp Demand	good solution	not applicable	may be helpful	not applicable	good solution	may be helpful	good solution	not applicable	may be helpful	good solution	not applicable	may be helpful
Weaving Sections	may be helpful	may be helpful	may be helpful	may be helpful	may be helpful	good solution	good solution	good solution	may be helpful	not applicable	not applicable	may be helpful
Lane Drops	good solution	not applicable	good solution	not applicable	good solution	good solution	good solution	good solution	may be helpful	may be helpful	may be helpful	may be helpful
Tunnels and Bridges	not applicable	not applicable	not applicable	not applicable	not applicable	good solution	not applicable	may be helpful	good solution	not applicable	not applicable	may be helpful
Horizontal and Vertical Curves	good solution	not applicable	good solution	good solution	good solution	not applicable	good solution	may be helpful	good solution	may be helpful	may be helpful	may be helpful
Narrow Lanes and Lateral Obstruction	may be helpful	may be helpful	may be helpful	may be helpful	may be helpful	not applicable	may be helpful	may be helpful	good solution	may be helpful	may be helpful	may be helpful
Inadequate Accelerated and/or Decelerated Lanes	good solution	good solution	good solution	not applicable	good solution	may be helpful	may be helpful	may be helpful	good solution	may be helpful	good solution	good solution

Source: Adapted from interim materials from NCHRP Project 3-83.

6. Speed Harmonization – the practice of utilizing monitored speed and volume data to adjust speed limits when congestion thresholds are exceeded and congestion and queue forming is impending. This mostly European practice reduces the traffic "shock wave" that results through congested corridors, and has an indirect benefit to bottlenecks and chokepoints. This practice requires overhead gantries.
7. "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.
8. Improving traffic signal timing on arterials.
9. Improving arterial corridors using access management principles.
10. High Occupancy Vehicle lanes or reversible lanes.
11. Providing traffic diversion information.
12. Implement road pricing to bring supply and demand into alignment. As public acceptance grows and legislative restrictions are relaxed, pricing will increasingly be viewed by transportation practitioners as a powerful and relatively easy way to implement strategy to address bottleneck congestion.

In 2006, as part of the research conducted for National Cooperative Highway Research Program Project 3-83 ("Low-Cost Improvements for Recurring Freeway Bottlenecks") a series of interviews with state and local transportation personnel occurred. Interviewed representatives were asked to name the low-cost improvements that their agencies have used at bottleneck locations in their jurisdictions. Table 3 was developed from these responses. The results showed that agencies are using a wide range of strategies to improve bottlenecks, most of them low-cost improvements that can be implemented quickly. Strategies include making creative use of existing highway geometry as well as selective additions to it.

The most frequently mentioned low-cost bottleneck improvements either analyzed or implemented by the interviewed agencies were:

• Ramp metering (7 responses);
• Auxiliary lanes (6); and
• High Occupancy Vehicle (HOV) lanes (4).

In general, the interview responses to this question suggest there is a trend towards favoring the implementation of ramp metering as a low-cost bottleneck improvement. Such systems are particularly attractive since they allow traffic managers to directly control freeway demand levels. This unique capability provides a valuable tool for managing freeway bottlenecks that does not involve large and expensive capacity expansions.

Some of the key questions and considerations in the development of improvement alternatives for bottleneck removal include:

• Is there an inside shoulder that would create a usable traffic lane for a short section of freeway?
• If there are bridges, are they wide enough to accommodate the extra lane while allowing adequate clearance to barriers (2 feet) and an outside shoulder? If not, are they short enough that a loss of shoulder as a breakdown lane would not be critical (500 feet or less)?
• If changes to an entrance or exit ramp or weaving area are considered, will adjusting the position of ramp gores cause geometric problems which must be resolved?
• Are vertical clearance issues, grade-matching, and sight distance problems created?
• If a shoulder is considered for conversion, is there right-of-way (ROW) to allow adding one back for part of the length of the project?
• If the bottleneck movement itself cannot be fixed reasonably, can the other traffic which is affected by it be better accommodated?
• Finally, will the improvement invite enough new traffic to cause immediate breakdown again or is this truly the clearing up of a "kink" in the system, without being a capacity addition which will overload some other part of the facility? (These options quoted directly from recent work by the Texas Transportation Institute: Freeway Bottleneck Analysis Methodology.)