Recurring Traffic Bottlenecks: A Primer
Focus on Low-Cost Operational Improvements
When Did “Plan on Being Delayed” Become Part of Our Everyday Lexicon?
The delays arising from traffic congestion seem an unavoidable frustrating fact of life. Or are they – unavoidable, that is? Why must we accept to allow 30 minutes for what should be a 15 minute drive? In today’s world, motor trips increasingly factor in dwell time to sit in traffic delay that is caused not by us, mind you, but by “others” who, if they would only get out of our way, would free up that trip to its rightful duration.
Most every driver, at some point or another, has experienced the frustration of traffic congestion. Congestion is caused by many factors, including physical bottlenecks – locations on the highway system where the physical layout of the roadway cannot process the traffic that wants to use it. While many of the nation’s bottlenecks can only be addressed through costly major construction projects, there is a significant opportunity for the application of operational and low-cost infrastructure solutions to bring about relief at these chokepoints. This document, Traffic Bottlenecks: A Primer on Low-Cost Operational Improvements, describes such facility breakdowns and explores the opportunity for near-term operational and low-cost construction opportunities to correct them. This document is the third-generation Primer in a series of advancing bottleneck activities, and is a key resource for Federal Highway Administration’s Localized Bottleneck Reduction (LBR) Program, which provides a virtual forum for peer exchange between members of the transportation community interested in alleviating bottleneck congestion. The LBR program, initiated in 2006, is designed to expand the portfolio of bottleneck reduction tools available to transportation agencies to encompass innovative, readily adopted strategies for reducing congestion at bottleneck locations.
Unclog That Bottleneck!
The root cause of recurring congestion is in fact bottlenecks and not necessarily uniform highway segments. We need to fix the “bends, kinks, and cavitations” in the traffic “plumbing” before
deciding to build more and bigger highways.
Traffic, Like Weather, is an Ever-Evolving “Front”
According to a February 2007 Harris Poll, one-quarter of respondents say traffic congestion is a serious problem that is not being addressed. But please don’t tell the thousands of practicing traffic engineers, planners, and road workers. It’s just that much like weather forecasting, traffic management is a dynamic moving target that makes it an ever-evolving profession. And like weather forecasting, we are getting better and better at it, but remain at the whim of unrelenting “fronts.”
With increasing attention, transportation professionals have come to realize that highway bottlenecks – specific points on the highway system where traffic flow is restricted due to geometry, lane-drops, weaving, or interchange-related merging maneuvers – demand special attention. Bottlenecks are localized sections of highway where traffic experiences reduced speeds and delays due to physical restrictions, too much demand, or both. The most severe ones tend to be freeway-to-freeway interchanges or systemic congestion, i.e., a corridor- or region-sized problem. However, many recurring bottlenecks are small, localized “hot spots” that may only require minor improvements. Examples include lane narrowing, short acceleration ramps, abrupt changes in highway design, and traffic signal deficiencies. To bring focus to this type of congestion, FHWA has established the LBR Program to promote the benefits of low-cost, quick-response solutions to recurring, localized bottlenecks.
The 2010 Congestion Report defines “Travel Time Index” (TTI) and “Planning Time Index” (PTI) as two measures of how congestion affects one’s on-road experience; namely, that a free-flow trip of a certain time will take several minutes longer under congested conditions (see box). The fact that a trip takes longer under congested conditions is not a startling concept, but the purpose of the annual report is to present an objective, data-measured comparison of how congestion is increasing, or in some cases receding, due to a constantly changing menu of causes and/or mitigation techniques. Indeed, the 2009 report showed declines in these measures for the prior two years. Performance and trend data like those presented in the Congestion Report will be a prerequisite as the highway transportation community moves towards adopting a Performance Management approach to selecting and funding projects. What is Performance Management? In a nutshell, it is monitoring the performance of the highway system in a variety of “goal areas,” evaluating projects to see what has been successful – or not – and using that knowledge to plan for future improvements.
Travel Time Index (TTI)
The TTI is one of the primary metrics used to measure congestion. It is the ratio of the actual travel time divided by the travel time under free flow conditions. A TTI of 1.2 means that a trip takes 20 percent longer than it would under ideal conditions.
Planning Time Index (PTI)
The PTI is a measurement of travel time reliability, which tell us how travel times for the same trip vary from day to day because of disruptions like incidents, bad weather, and work zones.
Source: 2010 Urban Congestion Trends Report.
“Bottlenecks” and not “Insufficient Facilities” is Increasingly the Problem
In the past, recurring congestion was felt to be a systemic problem (“not enough lanes”). It is true that additional lanes are usually needed in conjunction with bottleneck improvements to handle the additional traffic that is now freed up, but the root cause of recurring congestion is in fact bottlenecks, not uniform highway segments. Exhibit 1 shows these subordinate locations. Traditional capital solutions grew from this mindset, resulting in extensive corridor-wide improvements. The problem is that funding for these large scale projects is limited and they take a long time (many years) to complete, so recurring congestion goes untreated until funding becomes available.
However, if agencies shift their focus from recurring congestion being systemic (and thus treatable with only large projects) to being caused by specific chokepoints, a wider range of improvement strategies are possible, especially in the short term. While these will never replace the need for corridor-wide fixes – especially at the “megabottlenecks” such as freeway-to-freeway interchanges – bottleneck-specific improvements can provide effective congestion relief.
The recent economic downturn has caused a major shortfall in revenues to transportation agencies due to reduced tax collections. The low-cost nature of LBR strategies has made them highly attractive alternatives to traditional large-scale capacity expansion projects for agencies seeking “to do more with less.” Especially when combined with other low-cost operations and demand management strategies, LBR strategies are a major tool for addressing congestion cost effectively.
Versions 1 and 2 of the Bottleneck Primer introduced, and then raised the level of awareness about how LBR programs could deal with congestion, respectively. This Primer constitutes “Version 3” and is focused on providing highly specific guidance for agencies to follow in developing and advancing LBR programs.
Exhibit 1. Common Locations for Localized Bottlenecks
||Bottlenecks can occur at lane drops, particularly midsegment where one or more traffic lanes ends or at a low-volume exit ramp. They might occur at jurisdictional boundaries, just outside the metropolitan area, or at the project limits of the last megaproject. Ideally, lane drops should be located at exit ramps where there is a sufficient volume of exiting traffic.
||Bottlenecks can occur at weaving areas, where traffic must merge across one or more lanes to access entry or exit ramps or enter the freeway main lanes. Bottleneck conditions are exacerbated by complex or insufficient weaving design and distance.
||Bottlenecks can occur at freeway on-ramps, where traffic from local streets or frontage roads merges onto a freeway. Bottleneck conditions are worsened on freeway on-ramps without auxiliary lanes, short acceleration ramps, where there are multiple on-ramps in close proximity and when peak volumes are high or large platoons of vehicles enter at the same time.
|Freeway Exit Ramps
||Freeway exit ramps, which are diverging areas where traffic leaves a freeway, can cause localized congestion. Bottlenecks are exacerbated on freeway exit ramps that have a short ramp length, traffic signal deficiencies at the ramp terminal intersection, or other conditions (e.g., insufficient storage length) that may cause ramp queues to back up onto freeway main lanes. Bottlenecks could also occur when a freeway exit ramp shares an auxiliary lane with an upstream on-ramp, particularly when there are large volumes of entering and exiting traffic.
||Freeway-to-freeway interchanges, which are special cases on on-ramps where flow from one freeway is directed to another. These are typically the most severe form of physical bottlenecks because of the high traffic volumes involved.
|Changes in Highway Alignment
||Changes in highway alignment, which occur at sharp curves and hills and cause drivers to slow down either because of safety concerns or because their vehicles cannot maintain speed on upgrades. Another example of this type of bottleneck is in work zones where lanes may be shifted or narrowed during construction.
||Bottlenecks can occur at low-clearance structures, such as tunnels and underpasses. Drivers slow to use extra caution, or to use overload bypass routes. Even sufficiently tall clearances could cause bottlenecks if an optical illusion causes a structure to appear lower than it really is, causing drivers to slow down.
|Narrow Lanes/Lack of Shoulders
||Bottlenecks can be caused by either narrow lanes or narrow or a lack of roadway shoulders. This is particularly true in locations with high volumes of oversize vehicles and large trucks.
|Traffic Control Devices
||Bottlenecks can be caused by traffic control devices that are necessary to manage overall system operations. Traffic signals, freeway ramp meters, and tollbooths can all contribute to disruptions in traffic flow.