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The U.S. Government does not endorse products or manufacturers. Trademarks or manufacture

names may appear in this report only because they are considered essential to the objective 

the document.

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Form DOT F 1700.7 (8-72) Reproduction of completed pages aut

9. Performing Organization Name and Address

Cambridge Systematics, Inc.

1265 Kensington Drive

Knoxville, Tennessee 37922

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

DTFH-61-D-00004

12. Sponsoring Agency Name and Address

U.S. Department of Transportation

Federal Highway Administration

Offi ce of Operations (HOP)

1200 New Jersey Avenue, SE

Washington, DC 20590

13. Type of Report and Period Cov

Final Report, April 2012

14. Sponsoring Agency Code

15. Supplementary Notes

Neil C. Spiller, Transportation Specialist, FHWA, Offi ce of Operations (HOP), (202) 366-2188, neil.spiller@dot.gov

16. Abstract

This is the updated, 3rd version of this document. The fi rst version is FHWA-HOP-07-130, and the second version is FH

HOP-09-037.

While many of the nation’s bottlenecks are addressed through costly major construction projects (i.e., “megaprojects”) or 

transportation alternative solutions (e.g., HOV or HOT lanes, dynamic pricing, investments in transit alternatives, parking 

commuter incentive programs, et al.) there is a signifi cant opportunity for the application of operational and low-cost “fi x

spot-specifi c locations.

This Primer is the signature product of the Localized Bottleneck Reduction (LBR) Program, which is administered out of 

Offi ce of Operations, Offi ce of Transportation Management, at FHWA HQ in Washington, D.C. The LBR program is fo

on relieving recurring congestion chokepoints (as opposed to nonrecurring congestion causes) and the operational infl uen

cause them. Widening, lengthening, retiming, metering, or bypassing these problem areas to unclog them can often be do

lower cost, less intensive “footprint” means than traditionally waiting for a complete facility rebuild or an out-year project

In much the same way that transportation agencies might have an annualized safety-spot improvement program, e.g., a “to

list” of high accident locations, so too should they have an annualized congestion-spot program. If the ultimate fi x need 

complete facility overhaul, then so be it; but an agency needn't limit itself to only “building our way out of congestion.” 

17. Key Word

bottleneck, chokepoint, recurring congestion, low cost 

improvements, operational defi ciencies, operational infl uences, 

lane drops, weaves, merges, metering

18. Distribution Statement

No restrictions

19. Security Classif. (of this report)

Unclassifi ed

20. Security Classif. (of this page)

Unclassifi ed

21. No. of Pages

49

22. Price

N/A
i 

Introduction ........................................................................

When Did “Plan on Being Delayed” Become Part of Our Everyday Lexicon? ................

Traffi c, Like Weather, is an Ever-Evolving “Front” .............................................................

“Bottlenecks” and not “Insuffi cient Facilities” is Increasingly the Problem .....................

Understanding Bottlenecks ..............................................

What Exactly is a “Traffi c Bottleneck”? .................................................................................

“Good News” and “Bad News” About Fixing Bottlenecks .................................................

Understanding Merging at Recurring Bottlenecks ..............................................................

Merge Principles .......................................................................................................................

What is FHWA Doing to Promote Congestion Relief 

and Bottleneck Mitigation? .............................................

Recurring Congestion Program Strategies ............................................................................

Nonrecurring Congestion Program Strategies .....................................................................

The Localized Bottleneck Reduction Program  Focus on Recurring Congestion ..........

Benefi ts of Localized Bottleneck Improvements ..................................................................

Getting Started: How to Structure a 

Localized Bottleneck Reduction Program .....................

What is Stopping Us From Fixing Bottlenecks? ...................................................................

Overcoming Challenges to Implementing LBR Projects .....................................................

Options for Structuring a Bottleneck Improvement Program ............................................

Potential Issues with LBR Bottleneck Treatments ...............................................................

Identifying and Assessing Bottlenecks .........................

Where are the Bottlenecks and How Severe Are They? ......................................................

Localized Bottleneck Reduction Strategies ...................

Types of LBR Treatments ........................................................................................................

Innovative Intersection and 

Interchange Design Treatments ..............................................................................................

Success Stories: How Agencies Are Deploying 

LBR Treatments and Developing Programs .................

Case Studies ..............................................................................................................................

Successful LBR Program Development .................................................................................
ii 

Defi nitions ......................................................................................40

FHWA Matrix of Bottleneck Terms ...........................................42

Exhibits

 Exhibit 1. Common Locations for Localized Bottlenecks ....................................................................4

 Exhibit 2. The Worst Physical Bottlenecks in the United States (2010) .............................................6

 Exhibit 3. Typical Section of MN I-35W Northbound Priced Dynamic Shoulder Lane (PDSL) ..12

 Exhibit 4. Examples of How Agencies Have Addressed Localized Bottleneck Issues .................19

 Exhibit 5. MnDOT Project Screening Process .....................................................................................24

 Exhibit 6. Using Freeway Detector for Bottleneck Analysis .............................................................25

 Exhibit 7. Using Vehicle Probe Data for Bottleneck Analysis ..........................................................28

 Exhibit 8. Vehicular Movements at a Continuous Flow Intersection ..............................................31

 Exhibit 9. Crossover Movement in a DCD Interchange ....................................................................32

 Exhibit 10. Successful LBR Treatments  Austin, Texas......................................................................33

 Exhibit 11. Successful LBR Treatments  Arvada, Colorado ..............................................................34

 Exhibit 12. Successful LBR Treatments  Saginaw, Michigan ............................................................34

 Exhibit 13. Successful LBR Treatments  Metroplan MPO/Little Rock ...........................................35

 Exhibit 14. Successful LBR Treatments  Pittsburgh, Pennsylvania .................................................35

 Exhibit 15. Success Spawns Success .......................................................................................................36
Introduction

When Did “Plan on Being Delayed”

Become Part of Our Everyday Lexicon?

The delays arising from traffi c 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 traffi c 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 

traffi c 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 traffi c that wants to use it. While many of the nation’s bottlenecks can only be addressed through costly major construction projects, there is a 

signifi cant opportunity for the application of operational and low-cost infrastructure solutions to bring about relief at these chokepoints. This document, Traffi c 

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.

Traffi c, Like Weather, is an Ever-Evolving “Front”

According to a February 2007 Harris Poll, one-quarter of respondents say traffi c 

congestion is a serious problem that is not being addressed. But please don’t tell 

the thousands of practicing traffi c engineers, planners, and road workers. It’s just 

that much like weather forecasting, traffi c 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.”

Unclog That Bo

 The root cau

recurring conge

fact bottlenecks 

necessarily unifor

segments. We 

fi x the “bends, k

cavitations” in t

“plumbing” 

deciding to bu

and bigger hig
2

restricted due to geometry, lane-drops, weaving, or interchange-related m

maneuvers – demand special attention. Bottlenecks are localized sectio

way where traffi c experiences reduced speeds and delays due to physica

too much demand, or both. The most severe ones tend to be freeway-t

interchanges or systemic congestion, i.e., a corridor- or region-sized pro

However, many recurring bottlenecks are small, localized “hot spots” th

require minor improvements. Examples include lane narrowing, short a

tion ramps, abrupt changes in highway design, and traffi c signal defi cien

bring focus to this type of congestion, FHWA has established the LBR 

promote the benefi ts of low-cost, quick-response solutions to recurring

bottlenecks (http://www.ops.fhwa.dot.gov/bn/index.htm).

The 2010 Congestion Report (http://www.ops.fhwa.dot.gov/public

fhwahop11024/index.htm) defi nes “Travel Time Index” (TTI) and “P

Index” (PTI) as two measures of how congestion affects one’s on-road 

namely, that a free-fl ow trip of a certain time will take several minutes l

congested conditions (see box). The fact that a trip takes longer under 

conditions is not a startling concept, but the purpose of the annual rep

ent an objective, data-measured comparison of how congestion is incre

some cases receding, due to a constantly changing menu of causes and/

tion techniques. Indeed, the 2009 report showed declines in these meas

prior two years. Performance and trend data like those presented in the 

tion Report will be a prerequisite as the highway transportation commu

towards adopting a Performance Management approach to selecting an

projects. What is Performance Management? In a nutshell, it is monito

performance of the highway system in a variety of “goal areas,” evalua

to see what has been successful – or not – and using that knowledge to 

future improvements. 

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 

fl ow 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.
Insuffi cient 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 traffi c 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 specifi c chokepoints, a wider range of improvement strategies are possible, especially in the short 

term. While these will never replace the need for corridor-wide fi xes – especially 

at the “megabottlenecks” such as freeway-to-freeway interchanges – bottleneckspecifi c 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 specifi c guidance 

for agencies to follow in developing and advancing LBR programs. 
4

Location Symbol Description

Lane Drops Bottlenecks can occur at lane drops, particularly midsegment where one or more traffi c lanes ends or at a low-volu

ramp. They might occur at jurisdictional boundaries, just outside the metropolitan area, or at the project limits of 

megaproject. Ideally, lane drops should be located at exit ramps where there is a suffi cient volume of exiting traffi 

Weaving Areas Bottlenecks can occur at weaving areas, where traffi c must merge across one or more lanes to access entry or exit 

enter the freeway main lanes. Bottleneck conditions are exacerbated by complex or insuffi cient weaving design an

Freeway On-Ramps Bottlenecks can occur at freeway on-ramps, where traffi c from local streets or frontage roads merges onto a freew

Bottleneck conditions are worsened on freeway on-ramps without auxiliary lanes, short acceleration ramps, where 

multiple on-ramps in close proximity and when peak volumes are high or large platoons of vehicles enter at the sa

Freeway 

Exit Ramps

Freeway exit ramps, which are diverging areas where traffi c leaves a freeway, can cause localized congestion. Bottl

exacerbated on freeway exit ramps that have a short ramp length, traffi c signal defi ciencies at the ramp terminal in

or other conditions (e.g., insuffi cient storage length) that may cause ramp queues to back up onto freeway main lan

Bottlenecks could also occur when a freeway exit ramp shares an auxiliary lane with an upstream on-ramp, particu

there are large volumes of entering and exiting traffi c.

Freeway-to-Freeway 

Interchanges

Freeway-to-freeway interchanges, which are special cases on on-ramps where fl ow from one freeway is directed to 

These are typically the most severe form of physical bottlenecks because of the high traffi c volumes involved.

Changes in Highway 

Alignment

Changes in highway alignment, which occur at sharp curves and hills and cause drivers to slow down either becaus

concerns or because their vehicles cannot maintain speed on upgrades. Another example of this type of bottlene

work zones where lanes may be shifted or narrowed during construction.

Tunnels/

Underpasses

Bottlenecks can occur at low-clearance structures, such as tunnels and underpasses. Drivers slow to use extra caut

use overload bypass routes. Even suffi ciently tall clearances could cause bottlenecks if an optical illusion causes a 

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 tr

locations with high volumes of oversize vehicles and large trucks.

Traffi c 

Control Devices

Bottlenecks can be caused by traffi c control devices that are necessary to manage overall system operations. Traffi 

freeway ramp meters, and tollbooths can all contribute to disruptions in traffi c fl ow.
What Exactly is a “Traffi c Bottleneck”?

Webster’s Dictionary defi nes a “bottleneck” as: i) a narrow or obstructed portion 

of a highway or pipeline, or ii) a hindrance to production or progress. Certainly 

the elemental characteristics of traffi c bottlenecks exist in these descriptions. However, a road does not necessarily have to “narrow” for a bottleneck to exist (e.g., 

witness bottlenecks caused by a weave condition, sun glare, or a vertical climb). A 

bottleneck is distinguished from congestion in that it occurs at a specifi c location, 

and not pervasively along the entire corridor. 

Traffi c bottlenecks (hereafter, bottlenecks) have a myriad of causes. The most 

egregious ones tend to be freeway-to-freeway interchanges, but we all know that 

smaller, lesser chokepoints are frustrating too. Bottlenecks can be areas where traffi c is merging, diverging, or weaving, or where other physical restrictions exist like 

narrow lanes, lack of shoulders, steep grades, and sharp curves. The fact that many 

recurring locations are “facility determinate,” i.e., the design condition contributes 

to the resulting congestion. Facility design is a tangible feature that can always 

be improved; however the cost or the necessary right-of-way may be prohibitive. 

Alternately, demand can be reduced so that the bottleneck performs better. The 

LBR program is focused on the infrastructure side.

“Good News” and “Bad News” About Fixing Bottlenecks

The FHWA estimates that 40 percent of all congestion nationwide can be attributed to recurring bottlenecks (i.e., inadequate physical capacity) and another 5 

percent is attributable to ineffi cient traffi c signalization. The good news is that 

all these things are potentially correctable with mitigation strategies and roadway 

improvements. The bad news is that there are many, many candidate locations, and 

agencies are fi scally constrained on how much they can do. A tabulation of the top 

25 bottlenecks, compiled by INRIX in the National Traffi c Scorecard 2010 Annual 

Report, is shown in Exhibit 2. Their analysis uses raw data which comes from their 

historical traffi c data warehouse along with discrete “GPS-enabled probe vehicle” 

reports from vehicles traveling the nation’s roads – including taxis, airport shuttles, 

service delivery vans, long-haul trucks, and consumer vehicles. 
6

2010 

Rank Area Road/Direction Segment/Interchange State

Length

(Miles)

Hours 

Congested

1 New York Cross Bronx Expressway WB/I-95 SB Bronx River Parkway/Exit 4B NY 0.35 116 

2 New York I-95 NB U.S. 9/U.S. 1/U.S. 46/Exit 72 NJ 0.42 109 

3 Chicago Dan Ryan Expressway/I-90/I-94 WB Canalport Avenue/Cermak Road/

Exit 53

IL 0.40 105 

4 New York Cross Bronx Expressway WB/I-95 SB I-895/Sheridan Expressway/Exit 4A NY 0.51 133 

5 New York Cross Bronx Expressway WB/I-95 SB White Plains Road/Exit 5 NY 0.28 105 

6 New York Harlem River Drive SB 3rd Avenue NY 0.16 98 

7 Chicago Dan Ryan Expressway/I-90/I-94 WB Ruble Street/Exit 52B IL 0.12 115 

8 Chicago Dan Ryan Expressway/I-90/I-94 WB 18th Street/Exit 52C IL 0.41 107 

9 New York Cross Bronx Expressway WB/I-95 SB Westchester Avenue/Exit 5 NY 1.15 91 

10 Los Angeles Hollywood Freeway/U.S. 101 SB Vermont Avenue CA 0.62 117 

11 Los Angeles San Diego Freeway/I-405 NB I-10/Santa Monica Freeway CA 1.23 91 

12 New York Harlem River Drive S 2nd Avenue/125th Street/Exit 19 NY 0.22 110 

13 Chicago Kennedy Expressway/I-90/I-94 EB Ohio Street/Exit 50B IL 0.38 100 

14 Chicago Dan Ryan Expressway/I-90/I-94 WB Roosevelt Road IL 0.22 111 

15 New York Van Wyck Expressway/I-678 NB Hillside Avenue/Exit 6 NY 0.12 103 

16 New York Van Wyck Expressway/I-678 NB Liberty Avenue/Exit 4 NY 0.52 86 

17 Chicago Kennedy Expressway/I-90/I-94 EB Lake Street/Exit 51A IL 0.43 107 

18 Los Angeles Hollywood Freeway/U.S. 101 NB Alameda Street CA 0.27 102 

19 Los Angeles Hollywood Freeway/U.S. 101 NB Spring Street CA 0.14 110 

20 San Francisco CA 24 WB Gateway Boulevard/Exit 7A CA 1.12 66 

21 New York Harlem River Drive NB Lower Level Washington Bridge NY 0.11 108 

22 New York I-95 NB NJ 4 NY 0.81 81 

23 New York Major Deegan Expressway/I-87 NB 153rd Street/River Avenue/Exit 6 NY 0.29 79 

24 Los Angeles Hollywood Freeway/U.S. 101 SB Melrose Avenue CA 0.35 97 

25 New York Gowanus Expressway/I-278 EB NY 27/Prospect Expressway/Exit 24 NY 1.32 107 

Source: INRIX National Traffi c Scorecard 2010 Annual Report. http://scorecard.inrix.com/scorecard/Top100Bottlenecks.asp.

Exhibit 2. The Worst Physical Bottlenecks in the United States (2010) 
This guidance document focuses on “localized” recurring bottlenecks (i.e., pointspecifi c or short corridors of congestion due to decision points such as on- and 

off-ramps, merge areas, weave areas, lane drops, tollbooth areas, and traffi c areas); 

or design constraints such as curves, climbs, underpasses, and narrow or nonexistent shoulders.

The Difference in Merging for Recurring and Nonrecurring Conditions

Merging maneuvers at recurring bottlenecks are essentially “cat herding” with 

implicit rules (often local in culture or habit) at best. Typically, not much guidance is given – everyone is “on their own.” Drivers “suddenly” encounter taillights 

ahead and slow, then “swarm” to get past it, whereas, in a nonrecurring event, 

there is more apt to be advance warning and instruction in the form of orange 

cones, signs, fl agmen, or police. There is often direction to motorists how (“Take 

Turns”), where (“Merge Here”), and even what (“All Lanes Thru”) to do/expect, 

and there can even be enforcement (of lane jumpers) or simply order (traffi c cops) 

from chaos. One might argue “What’s the difference? I’m in bumper-to-bumper 

traffi c regardless!” The great difference is the greater potential (in nonrecurring) 

for herding those cats. 

Controlling the chaos of lane merging is fundamental to advanced traffi c operations strategies. Ramp metering has long been used to limit the number of merges 

at a recurring bottleneck in order to prevent breakdown of traffi c fl ow. In nonrecurring situations the “dynamic lane merge” or “lane control” is increasingly 

used where an incident or work zone has “stolen” a lane. This strategy proactively 

directs motorists to both slow down and to get into the appropriate travel lane well 

in advance of the problem. Active Traffi c Demand Management (ATDM) strategies take and advantage of lane control as well as other types of actions to balance 

demand and available capacity. Several U.S. examples of this strategy already exist 

and more are planned: I-35W in Minneapolis and U.S. 2 in Seattle have functioning 

systems. I-66 in Virginia has an older system in place that will be upgraded over 

the next few years

Which is Best? “Early” or “Late” Merging?

Can a better recurring merge be developed? Merging takes place at-speed or “atcrawl.” The former is most often associated with free fl ow on-ramp maneuvers, 

while the latter is most often associated with bumper-to-bumper congestion. In 

either condition the motorist has the additional choice to merge “early” (upstream) 

or “late” (at point of confl uence). This creates a matrix of four possible merge 

conditions; 1) at-speed “early;” 2) at-speed “late;” 3) at-crawl “early;” and 4) atcrawl “late.” To further complicate things, guidance concerning where, when, and 

So You Th

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8

Given that this Primer is focused on the recurring bottleneck genre, the 

of this section was to research if early or late merging was best for thes

trolled situations; i.e., when no active TCP exists.

What Instruction is Given to Motorists?

On the whole, drivers are typically left to their own strategies as to how to 

together at recurring chokepoints. Personal preference, impatience, frust

speed differentials, and other human and vehicular traits conspire to infl u

safety and reduce effi ciency. Altruistic drivers are unselfi sh and yield – in 

degrees – to proactive drivers, who seek only their own benefi t to cut in 

only real conclusion that can be drawn is suggested by the similarity in m

ologies used in the work zone studies. Specifi cally, in setting up “Dynam

Zones” these are essentially systems that are “on” when traffi c volumes a

“off ” when traffi c volumes are low. The mere fact that all of these trials 

to set up – and study – a “late merge” scenario speaks to the engineering 

nity’s penchant towards this method over the “early merge” option for sto

conditions. One theme, however, remained constant. Regardless of the 

forewarning and direction given to motorists (e.g., “light” guidance in rec

ations and “heavy” guidance in nonrecurring situations) personal preferen

to win the day. Absent absolute enforcement, motorists were observed to 

opined to – merge when and how they preferred, with less regard for any 

Early Attempts to Direct Motorists How to Merge

When the Interstates were built in the 1960s and 70s there was often “in

by local engineers and the media of how to engage Interstate ramps, ac

and deceleration lanes, etc. Of course, at that time, traffi c was less cong

the whole, and the merging and diverging were essentially lessons in how 

and exit Interstates. Academia has touched on queue theory, gap analys

related safety-oriented aspects, but none of these studies have focused m

educating motorists how to merge effi ciently, unless one considers a “qu

“traffi c stream” as an entity that can deduce instruction. Nevertheless, 

demic community has essentially confi rmed, via queuing theory and mi

tion that the discharge rate after the merge governs congestion on the s

In layman’s terms, there is a fi nite capacity of the single lane downstream 

constriction. Very little of what happens upstream can refute the laws o

that only one vehicle can occupy the discharge space at a time; and in a 

situation, the lead vehicle does so from essentially a crawl speed.

Excepting for some basic, generic instruction in states’ drivers manuals 

a safe gap in traffi c” – typ.) little has been done at the national level to e

“Close to half of all 

congestion happens day 

after day at the same time 

and location.”

Source: http://www.fhwa.dot.gov/

congestion/describing_problem.htm.
tion, or pedestrian rights and practices. The perceived reason for this may simply 

be the expectation that there will always be drivers who feel they know best how 

and when to merge in a queue, irrespective of any instruction to the contrary. The 

altruistic view is to leave gaps, yield to your neighbor, take your turn but don’t force 

your turn, and generally don’t deny him or her entry into your lane. The more proactive view is to take fi rst opportunity to cut in line, perhaps “line jump” to chase 

whichever line seems to be moving, and scuttle the principles of any orderly manner. Anecdotal evidence from many local traffi c blogs and an Internet search fi nds 

strong sentiment from both camps as to why they think their method is best.

Merge Principles

How can we increase the effi ciency of merging prior to the discharge point? In 

two words – be orderly. Not surprisingly, safety improves too. It is repeatedly 

shown that traffi c is inherently safer when all vehicles are traveling at or near the 

same speed. Think of an orderly progression on a crowded escalator. Everyone is safely cocooned because they are going the same speed. Now imagine the 

bumping and chaos that would occur if impatient folks pushed past others.

Principle #1: “Go Slow to Go Fast”

“Go slow to go fast” is an increasingly trendy expression in traffi c circles. It speaks 

to the seemingly paradoxical idea that if we slow down the rate of our “mixing” 

we can get past a constriction faster. A well known example (actually the winning 

entry in a 2006 contest to demonstrate the meaning of “throughput maximization”) is the “rice” experiment. In the fi rst case, dry rice is poured all at once into a 

funnel. In the second case, the same amount is poured slowly. Repeated trials generally conclude about a one-third time savings to empty the funnel via the second 

method. And, it should be noted, there is a tipping point reached as one graduates 

from a v-e-r-y slow pour, to a medium pace, and so on. What lesson does the rice 

experiment teach us about traffi c? The densely packed rice (or traffi c) in the fi rst 

trial creates friction in the literal sense and the practical sense, respectively. The 

denser the traffi c, the smaller the safety cushion around each driver, and the more 

cautious (i.e., slower) he becomes. A classic “bell curve” diagram also serves to 

explain how traffi c throughput reaches an apex up to the point where traffi c friction and confl ict conspire to begin a decline in the rate of throughput and speed. 

There exist some examples of validation of this principle at intersections (e.g., 

traffi c signalization, roundabouts, vehicle detection) that demonstrates that slowing 

or stopping some traffi c benefi ts the aggregate fl ow, and is far better than the freefor-all converse. In the bottleneck and corridor genres, we have ramp metering and 

speed harmonization, respectively, providing examples on freeways.
10

pg (y, gp) gp

traffi c fl ow. Gaps allow for small adjustments in braking, accelerating, and 

The larger the gap, the lesser the “ripple” affecting adjacent and following 

which otherwise would react by slowing. Gap maintenance (and thus, lane 

is achieved on-purpose in high-occupancy vehicle (HOV) lanes or high-oc

(HOT) lanes; by selective admittance in the former, and by dynamically sh

price every few minutes in the latter. The target benefi t is to allow qualify

the guarantee of a free fl ow trip, versus the hit-or-miss prospect in the 

general purpose (GP) lanes. Both cases have the added (and intended) 

of removing vehicles and or person-trips from the GP lanes too; so all 

streams win when these practices are employed. Absent out-and-out vi

who can muck up the system, agencies can tweak the lane mandates to 

systems running at optimum levels. How does this apply to localized bo

Theoretically, the same “gapping” principles would hold true in backup

leaving progressively larger gaps would allow for progressively better p

(Taken to the extreme, no “bottleneck” would even exist!) The point 

congested situations the constant brake-tapping in bumper-to-bumpe

works to self-perpetuate the problem. No one can get much momen

he or she has to react to the vehicle directly ahead or adjacent. The r

effects are short, abrupt, and ineffi cient. The obvious problem with 

human nature simply won’t allow for the patience and orderliness to 

work. The second that I create a suffi cient gap between me and the 

some “profi teering” lane jumper will fi ll it. Which is a nice segue int

principle; zippering. 

Principle #3: Zippering

Unlike principle #2, which is noted to be fairly impractical to expect, th

could easily be melded into our regular practice; namely, to take turns, o

per” merge at the front of the line. The fairness – and simple visualiza

this principle speaks for itself. And there is already precedence that we 

schooled in this; witness the “Yield” condition and many recurring locatio

this is the unwritten rule; newcomers quickly adapt! Advocates of zipper m

proponents of “late” merges; i.e., staying in your lane until the last possibl

and taking turns to get through the chokepoint nozzle. One enterprisin

California has gone so far as to adorn his car with a zipper graphic and 

promoting this method. 

Is Murphy Right? Does the Other Lane “Always Move Faster”?

How many times have you observed (or seemed to observe) that “the oth

moving faster” only to get into that lane and then watch the fi rst lane mo

Actually, you are at the whim of “observation selection bias” which ess

“Zippering”
less so the ones that aren’t. So, does cutting in line help you?

Imagine two lanes of cars. The left lane (L) is the continuous lane and the right 

lane (R) is dropping. You are 6th in line in R lane. If everyone stays put and “zippers” then the zipper order is L, R, L, R, etc. Your neighbor to your left is 11th

and you will be 12th to merge. If, however, you “early merge” and cut in front of 

him into the L line, then you will now be 11th to merge, the person behind you 

(formerly 14th) moves up to 12th, and you neighbor drops to 13th. You win. Your 

neighbor loses. But the guy behind you benefi ts most. 

Now consider the same scenario except the zipper order is R, L, R, L, etc. In the 

orderly scenario you would be 11th and your neighbor is 12th. If you cut in front 

of him, the guy behind you moves up to 11, you are now 12th, and your neighbor 

is now 14th. You neighbor really loses (drops two slots) and the guy behind you 

(formerly 13) really wins; he gains two spots – again.

Congratulations! In both scenarios you have defi nitely improved the slot for the 

guy behind you! You may or may not have improved your slot. And in either case, 

you made your neighbor mad! And in the end, all the jockeying you have done 

may have been canceled by someone ahead of you. So maybe it’s better to leave 

Murphy’s Law to “anything that can go wrong, will” and let zippering be the fair 

and simple solution to traffi c backups. 

Principles Put Into Practice: 

Variable Speed Limits and Speed Harmonization

Variable speed limits (mostly tried in work zones; i.e., nonrecurring conditions) and 

the European concept of “speed harmonization” (nonwork zones) both intend to 

“harmonize” traffi c by regulating speeds. In the latter case, a series of overhead 

gantries gradually adjust speeds through congested highway segments in order 

to fl atten the sinusoidal effect of traffi c speeds bouncing between open sections 

and interchanges. Speed harmonization is typically effected as the open highway 

approaches the denser central business district. A great expense is incurred by 

the cost of the overhead, spanned gantries, the necessary detectors, the interconnectivity, the necessary operational overhead, and the sheer number of gantries 

required along the multikilometer corridor. “Go slow” (harmonize) can therefore be used as a strategy as a means to move more traffi c than otherwise might 

have gotten by. Several tests of speed harmonization are in the planning stages 

throughout the United States. 

For example, the Minnesota DOT has deployed a variable speed limit system on 

I-35W in Minneapolis in conjunction with a “priced dynamic shoulder lane” (PDSL). 
12

g ppgp

42nd Street to downtown;

• Two-plus carpools, vanpools, transit, motorcycles travel toll free;

• Dynamically priced based on demand;

• PDSL operates as a priced lane during peak periods to maximize capacity on existing roadways;

• Electronic signs alert drivers whether the PDSL is open or closed; and

• Variable speed limits are set in the adjacent nontolled lanes.

Exhibit 3. Typical Section of MN I-35W Northbound Priced Dynamic Shoulder Lane (PDSL

Minnesota I-35W Planned Overhead Signage Showing 

Priced Dynamic Shoulder Lane and Variable Speed Limits

PDSL Opened PDSL Closed

Source: MnDOT.

Source: Simulated Photos.
Promote Congestion Relief 

and Bottleneck Mitigation?

With regard to congestion, the Federal Highway Administration (FHWA) promotes a number of efforts to help reduce congestion on the nation’s highways. 

Together with our state partners, who implement these strategies, these efforts 

can allow for more informed decisions, better coordination, and quicker actions to 

mitigate the problems.

Recurring Congestion Program Strategies

Tolling and Pricing. Value pricing entails fees or tolls for road use which vary by 

level of vehicle demand on the facility. Fees are typically assessed electronically to 

eliminate delays associated with manual toll collection facilities.

Public-Private Partnerships. Public-private partnerships (PPP) refer to contractual agreements between a public agency and private sector entity that allow 

for greater private sector participation in the delivery of transportation projects. 

FHWA is working with our partners in the public and private sector to further 

investigate these promising partnerships.

Real-Time Traveler Information. This is “decision-quality” information that 

travelers can access, understand, and act on to choose the most effi cient mode 

and route to their fi nal destination. Timely and detailed information about traffi c 

incidents, the weather, construction activities, transit and special events, all aid in 

improving travel time predictability, better choices, and reduced congestion.

Corridor Traffi c Management. When congested traffi c conditions occur on one 

roadway, travelers typically respond by shifting to another route, selecting a different roadway (freeway versus surface street), adjusting their trip to another time 

of day, or remaining on their current route encountering signifi cant delays. The 

proactive use of managed lane strategies, alternate routing of traffi c, and proactively managing and controlling traffi c within freeway corridors offer are a few 

useful approaches. 

Arterial Management and Traffi c Signal Timing. Signal timing should correspond to the current traffi c patterns. Often signals are initially timed, but not 

readjusted when traffi c patterns change. This results in ineffi ciency and unnecessary delays. Goal: work with state and local agencies in congested metropolitan 

areas and encourage best practices for improved traffi c signal timing.
14

sity, or safety conditions thereon. Active Traffi c Management (ATM) o

Transportation Demand Management (ATDM) are brother and sister t

wherein, the former is typically applied only to the roadway facilities, a

is typically a broader integration of a larger pool of related activities, li

parking, and driver-behavior elements. ATM enhancements involve so

“smart highway” feature that uses real-time speed, vehicle-count, or ev

occupancy data to open or close certain lanes, adjust the speeds on the 

or vary the candidacy to even be in certain lanes (e.g., HOV, HOT, truc

in the fi rst place. 

Nonrecurring Congestion Program Strategies

Traffi c Incident Management. This utilizes a combination of public 

functions and traffi c management functions – it requires cooperation b

ous public agencies to reduce congestion by clearing traffi c crashes and 

stalled vehicles. FHWA is championing laws, policies, and practices tha

the clearance of major and minor incidents that create congestion.

Work Zone Management. This program is working to “make work z

better” by providing transportation practitioners with high-quality prod

and information that can be of value in planning, designing, and implem

safer, more effi cient, and less congested work zones.

Road Weather Management. This program seeks to better understan

impacts of weather on roadways, and promote strategies and tools to m

those impacts.

Highways for LIFE. Highways for LIFE is all about building faster, s

better quality, less cost, and causing less work zone congestion. The pu

Highways for LIFE (HfL) is to advance longer-lasting highway infrastru

innovations to accomplish the fast construction of effi cient and safe hig

bridges. The three goals of HfL are to improve safety during and after 

tion, reduce congestion caused by construction, and improve the quality 

highway infrastructure.

The Localized Bottleneck Reduction Program – 

Focus on Recurring Congestion

In concert with the above focus areas, FHWA’s Localized Bottleneck Reduc

Program is entirely aimed at reducing recurring congestion. The LBR Pro

promotes operational and low-cost bottleneck mitigation strategies to improve m
points that are recurring events; ones that are wholly the result of operational infl uences. The goal of the program is to raise awareness of bottlenecks at the state level 

and promote low-cost, quick-to-implement geometric and operational improvements to address 

recurring chokepoints. The LBR Program has several activities underway, including:

• This Primer, which is in its third iteration, providing an overview of the wide 

range of operational and low-cost strategies available to reduce congestion at 

bottlenecks and provides guidance for agencies implementing LBR programs;

• A compendium of state best practices in bottleneck identifi cation, assessment, countermeasures, and evaluation, including how bottlenecks are treated 

in the annual planning and programming processes;

• Version X of the Traffi c Analysis Toolbox which focuses on focusing on 

what analysis tools are available, necessary and productive for localized congestion remediation; and

• State-specifi c workshops for state and local agencies to learn and share 

information on localized bottleneck reduction strategies and how they can be 

incorporated into their respective planning processes.

Benefi ts of Localized Bottleneck Improvements

The LBR Program focuses on operationally infl uenced bottlenecks – small, localized 

“hot spots” where the design of the roadway itself becomes the constricting factor 

in processing traffi c demand, resulting in recurring delays of generally predictable 

times and durations. Megaprojects required to resolve major bottleneck problems 

and systemic congestion (e.g., entire corridor rebuilds, multimile lane additions, 

and systemwide improvements) are far and above the focus of this program area. 

Unfortunately, when weighed against these larger, more visible projects, localized 

bottleneck problems often receive lower priority for funding or are put off entirely 

until they can be implemented as part of the larger, all-encompassing project. However, in this day and age of fi scal constraints, with agencies facing over-escalating 

costs and increasingly limited right-of-way, it is evident that “business as usual” in 

resolving congestion problems no longer applies. Low-cost bottleneck mitigations 

have several advantages that can help agencies deal with these developments:

• They address current problems and therefore have high visibility. 

Agencies are under increasing pressure to do something immediately about 

congestion problems. Because low-cost bottleneck treatments are small in 

scale, they can be implemented quickly, so benefi ts start accruing immediately.
16

weaving and merging areas, thereby increasing the cost-effectiven

to safety merits.

• They will be required as transportation funding for megapro

becomes more constrained. Major reconstruction projects are 

fi ed as the only valid solutions to relieve congestion at the worst 

locations. However, the cost of executing such projects is usually 

Low-cost bottleneck improvements provide an effective way to st

scarce resources.

• Lower cost means more locations can be addressed. More sp

can be implemented throughout a region, addressing more corrido

a few large projects.

• They are less invasive on the physical and human environme

environmental footprint of low-cost bottleneck projects is very lo

terms of disruptions during construction and fi nal design.

• They are not necessarily just short-term fi xes. For some low-

ments, congestion benefi ts will play out over many years, not just 

fact, when combined with other forms of treatment (e.g., demand 

ment and operations), they may be part of a long-term solution fo

location or corridor.

• They may be considered part of major reconstruction projec

address current problems. Some state DOTs have successfully 

rated low-cost bottleneck treatments within the context of larger, 

reconstruction projects.
Structure a Localized 

Bottleneck Reduction Program

What is Stopping Us From Fixing Bottlenecks?

States have cited a number of barriers to establishing bottleneck-specifi c or similar 

programs that target chokepoint congestion:

• A predisposition for large scale, long-term congestion mitigation projects. Traditional transportation planning and programming efforts are often 

predisposed toward major capital improvement projects to relieve congestion such as corridor-widening or massive reconstruction of an interchange. 

There is also no shortage of demand management strategies designed to fi ght 

the congestion battle, such as HOVs, tolling and pricing, transit alternatives, 

and ridesharing programs. But the onerous processes involved in many of 

these initiatives can squeeze out smaller programs.

• Lack of program identity. Unless there is a formal program identity, 

bottleneck remediation is usually relegated to a few projects completed as 

part of an annualized safety program, or as a subordinate part of larger, other 

purposed projects.

• Lack of a champion. Many successful state or metropolitan planning 

organization programs are the result of one or more persons taking charge 

to either mandate or adopt a program. High-level administrators often set 

the policy direction and strategic initiatives for their agencies, while midlevel 

managers’ production refl ects their priorities and skills in executing those 

initiatives.

• Lack of resources. Many state agencies are fi nding themselves overworked 

and understaffed. Although the return on investment for LBR projects are 

high, agencies often do not have the in-house resources necessary to conduct detailed analyses required to evaluate and prioritize the large number of 

potentially competing projects. With limited resources, agencies are relegated 

to hiring consultants and/or universities to conduct detailed project analysis.

• Lack of funding. With many state agencies experiencing major budget 

shortfalls, lack of funding continues to be an often cited barrier to implementing new programs. http://www.ops.fhwa.dot.gov/pu

fhwahop11034/index.htm
18

often included in the ongoing planning and programming processes fo

agencies. Others struggle with how best to identify problem locations, 

existing conditions, and quantify the impacts of proposed remedies, as 

is no structured process in place. For example, in developing their stru

LBR program, Michigan DOT cited challenges regarding how best to j

and evaluate project impacts while creating a level playing fi eld for appl

of LBR funding across each of their seven regions.

• A culture of historical practices. Many agencies face institutional ch

lenges in changing their current business practices. For example, one a

dutifully executed an annualized “safety” program and looked only at c

rates in determining their annual top 10 list of projects. After institutin

congestion mapping process, they identifi ed several signifi cant stand-al

chokepoints that did not correlate with their high-crash mapping. The

high-congestion hot spots competed with high-accident hot spots on th

unifi ed top 10 list of projects.

In addition, even if there is agreement that an LBR should exist, barriers oft

exist for implementing specifi c projects, including:

• Design challenges. LBR treatments may sometimes require “nonstan

designs. Seeking exceptions to design standards is often tedious with n

antee that they will be approved. 

• Safety challenges. Even if design issues are resolved, safety issues m

be present. For example, eliminating a shoulder to obtain an extra thro

lane may have safety implications.

Tackling the Challenges and Barriers to Fixing Bottlenecks

The FHWA publication, An Agency Guide on Overcoming Unique Challenges to Loc

Congestion Reduction Projects, provides more guidance for agencies wishing to im

ment an LBR program. This report presents and describes examples of inst

tional, design, funding, and safety challenges that agencies face when trying t

develop unique solutions to localized congestion problems. The main questi

that this guidance helps an agency address are: 

1. What are the most common barriers and challenges with addressing loc

congestion problems?

2. What are some case study examples that highlight how barriers and cha

were overcome?

3. What are some of the key factors in successful implementation of loca

bottleneck projects?

more about how 

to get an LBR 

program off 

the ground?

http://www.ops.fhwa.dot.gov/publications/

fhwahop11009/index.htm
Challenge Description Case Studies Outcome

Institutional

Having a project champion. Dallas, TX

Kansas City, KN

+: 20+ projects due to DOT/MPO champions

+: Governor passes bill allowing buses on shou

Disposition towards megaprojects. Minneapolis, MN

Manchester, NH

+: Similar benefi t for $7 versus $138 million pr

+: Expedited work at Exit 5 as part of megapr

Project planning and programming requirements. Danbury, CT

Austin, TX

+: Restriping at Exit 7 improved fl ow signifi can

+: Multidisciplinary group mitigating congestio

Lack of training/understanding on how to develop a successful project. Dallas, TX 

LBR workshops

+: Freeway Bottleneck Workshop.

+: Federal outreach workshops building consen

Knowledge of problem locations that can be fi xed with low-cost solutions. Phoenix, AZ

Dallas, TX 

Little Rock, AR

+: Regional bottleneck study. 

+: Aerial freeway congestion mapping. 

+: Operation Bottleneck program by MPO.

A culture of historical practices. Saginaw, MI +: Successful roundabout at I-75/M 81 interch

Defi ciency with internal and external coordination (design/operations). New York, NY +: PFI functional groups.

Can’t implement projects without being in approved regional/state plans. Rhode Island DOT +: Creation of the Strategically Targeted Affor

Solutions (STARS) program. 

No incentive or recognition for successful low-cost bottleneck reductions. Dallas, TX +: Engineers performance evaluation includes 

Will the proposed solution work – lack of confi dence. Florida DOT +: Trial fi x with cones made permanent with st

Design

Design exception (DE) process is diffi cult. Pittsburgh, PA +: New shoulder to avoid DE, Academy at I-2

“Nonstandard” design is considered a deal-breaker. Minnesota DOT +: Creation of “fl exible design” concept.

Problem is too big and nothing short of a rebuild will fi x it. Plano, TX +: Implement auxiliary lane on U.S. 75 at SH 1

Spot treatment will move problem downstream and not improve mobility. Renton, WA +: SR 167 spot fi x near Boeing reduces conges

Standard design practices contribute to bottleneck formation. Fort Worth, TX +: I-20/SH 360 fi x defi es AASHTO basic lane

Funding

There is no dedicated funding category for this type of project. Mississippi DOT

Nebraska DOT

+: I-10 shoulder use after Katrina improves fl o

+: ITS funds for ramp gates to fi x U.S. 75 bottl

Low-cost solution may blur or preclude need for bigger project. Dallas, TX +: I-635 early action doesn’t stop $3B megapro

Don’t understand if alternate funding categories can be used. Virginia DOT 

Ohio DOT

+: STARS program uses safety $ to target cong

+: Safety funds include congestion index.

Lack of available resources (e.g., DOT striping crews) for implementation. Dallas, TX +: District striping contract implements small fi 

Safety

Hesitancy to implement solution that does not follow standard design. Minnesota DOT +: Mobility crisis from I-35 bridge collapse.

Perception that safety is compromised with low-cost, nonstandard fi xes. Texas DOT +: Average 35 percent crash reduction for 13 p

Lack of shoulders takes away necessary refuge areas. Arlington, TX +: Crash reduction at SH 360/Division. 

Lanes that are not full width create safety issues for large trucks. Dallas, TX +: I-30 Canyon truck rollovers basically elimina

Exhibit 4. Examples of How Agencies Have Addressed Localized Bottleneck Issues

Through a series of case studies, documented in An Agency Guide on Overcoming Unique Challenges to Localized C

tion Reduction Projects, states and MPOs have developed innovative ways to overcome the common barriers to LBR 

(Exhibit 4.) The case studies identifi ed the most common barriers and challenges with addressing localized cong

lems and the key factors in successful implementation of localized bottleneck projects.
20

The goal of the FHWA’s Localized Bottleneck Reduction Program is to 

awareness of bottlenecks at the state level and promote low-cost, quick 

ment geometric and operational improvements to address recurring cho

There are no set guidelines for establishing an LBR program and no tw

will look the same. State DOTs, MPOs, or local transportation agencie

traditional organizations who lead LBR efforts as part of larger mission

organization. Many times, the state may identify bottlenecks and work 

MPOs to integrate these projects into the TIP and other targeted fundi

such as CMAQ and safety. Other times, low-cost bottlenecks can be ad

programmatically at the state DOT level by reviewing existing plans and 

and look for opportunities to include LBR improvements and strategies

Examples of how transportation agencies have structured LBR program

the following:

• Periodic Special Program or Initiative. For example, in 2007, t

nesota DOT was asked by the Legislature to develop a rapid turna

to identify low-cost, quickly implementable projects that were not 

identifi ed by the traditional planning and programming processes. 

of months, this unique approach led by the Traffi c Management C

neers basically “brainstormed” low-cost, candidate projects that w

problems, but for whatever reasons, had never landed on tradition

Improvement Programs. In 2008, the Central Arkansas MPO und

“Operation Bottleneck,” a campaign to openly solicit public input 

date locations, but one that has a fi nite life span. 

• Incorporating Bottlenecks into Other Programs. At the state 

level, low-cost bottlenecks can be addressed programmatically eve

special program or initiative. One approach is to conduct a review 

plans and look for opportunities to include LBR improvements in 

For example:

Ohio DOT added a congestion-based index ranking to their annual iden

of spot safety problems for the Federal Hazard Elimination Program (

Caltrans, as part of their Corridor Management Process, includes th

cation of bottlenecks and potential short-term fi xes as part of an ov

long-term strategy for making corridor improvements. 
• Formal Low-Cost Bottleneck Improvement Program (ongoing). 

Another option is to establish a defi ned bottleneck program within the 

agency. For example, Virginia DOT (VDOT) has implemented the Strategically Targeted Affordable Roadway Solutions (STARS) Program, which is a 

safety and congestion program that partners state, planning district and local 

transportation planners, traffi c engineers, safety engineers, and operations 

staff to identify “hot spots” along roadways where safety and congestion 

problems overlap and are suitable for short-term operational improvements. 

Following VDOT’s success, the Rhode Island DOT (RIDOT) created its own 

version of the STARS program to meet its low-cost bottleneck program needs. 

part of their project planning and development process. The recent Moving 

Washington initiative incorporates LBR concepts into a coordinated program 

to address congestion. At the planning stage in their Highway System Plan, 

WSDOT considers bottlenecks together with traditional corridor improvements under the “Congestion Relief ” category. Congestion relief projects are 

ranked using the benefi t/cost ratio, contribution to performance goals, and 

other qualitative factors, and compete on these bases with projects in other 

categories in the Highway System Plan: Preservation, Safety, Environmental 

Retrofi t, Economic Vitality, and Stewardship. 

At the metropolitan planning organization level, the short-term nature of LBR 

projects meshes well with the Congestion Management Process (CMP) and 

“planning for operations,” which are new initiative areas for planners. As planners’ perspectives broaden to include these short-term views of the system (in 

addition to the traditional long-range view), an LBR program makes perfect 

sense from a planner’s viewpoint, LBR improvements would be another aspect 

of the CMP process. Because an LBR program should be data- and performance-driven, it is a logical complement to a CMP; the same data should be 

used for both purposes. In fact, within the context of the CMP, it may useful to 

make the two processes seamless, at least at the MPO level.
22

to implanting LBR strategies also exist. Agencies have cited the followi

associated with LBR strategies:

• Compliance with State Implementation Plans (SIP) for Air 

Quality Conformity. SIPs set forth the state’s strategy for gettin

quality within National Ambient Air Quality Standards (NAAQS) 

it there. They include a large variety of project types, including tra

tion projects, and extensive emissions modeling is undertaken to e

their impact. There is a great deal of uncertainty as to how an LB

might affect the SIP: does the entire SIP have to be redone, does 

modeling just for the LBR project have to be performed, or can th

sions impacts be assumed to be small enough that they can be ign

occurrences must be dealt with on a case-by-case basis by agencie

undertake bottleneck projects. One point worth noting: if air qua

formity in a location precludes or discourages major capital expan

additional lane-miles), the type of improvements in a localized bo

program clearly do not fall in this category.

• Compliance with Long-Range “Design Concepts.” In some 

design concept or goal has been formally established for a roadwa

ridor. The thought is that any improvements should be part of th

cept. When the design concept is institutionalized, it may be diffi c

deviate from it with an LBR treatment that does not match. Agen

decide and weigh the benefi ts themselves whether the cost of doin

bottleneck solutions in the short term is against the cost of waitin

more complete solution. This decision can be diffi cult, especially 

cies without a good appreciation for the typical benefi ts and costs 

bottleneck solutions and how long those benefi ts might last.

• Compliance with Design and Safety Standards. LBR treatmen

be of a smaller scale than typical capital improvement projects. T

that the redesign is usually not made to existing design standards, 

depending on the funding source, may require a formal design exc

Further, even if a design exception is not needed, safety problems 

introduced by the LBR treatment, especially if the identifi ed prob

gestion-oriented. To address this issue, LBR treatments need to b

for potential safety impacts prior to implementation. Also, a Road

Audit of the design would be benefi cial. Based on the review, add

gation of safety impacts may be warranted, or a close monitoring 

experience at the site may be used. Finally, agencies should be in c

the FHWA Division offi ces throughout the process as design revi

required, depending on circumstances.
Where Are the Bottlenecks and How Severe Are They?

Every highway facility has decision points such as on- and off-ramps, merge areas, 

weave areas, lane drops, tollbooth areas, and traffi c signals; or design constraints 

such as curves, climbs, underpasses, and narrow or nonexistent shoulders. In 

many thousands of cases, these operational junctions and characteristics operate suffi ciently and anonymously; however, when the design itself becomes the 

constricting factor in processing traffi c demand, then an operationally infl uenced 

bottleneck can result.

The degree of congestion at a bottleneck location is related to its physical design. 

Some operational junctions were constructed years ago using design standards 

now considered to be antiquated, while others were built to suffi ciently high design 

standards but are simply overwhelmed by traffi c demand. The following sections 

provide some guidance on how to identify bottleneck locations.

Direct Observation

At the local level, engineers and planners are often aware of problem locations 

because they can directly observe the congestion they cause. Soliciting the input of 

local transportation personnel has been used successfully by many States in identifying bottleneck locations. Once the locations are identifi ed, the nature of the 

problem can be assessed. Exhibit 1 on Page 4 previously presented some examples 

of the types of geometric and highway features related to bottlenecks; these can be 

used as a screen to identify the specifi c problem that causes the bottleneck. 

Minnesota DOT successfully combined expert judgment, data analysis, and modeling to develop a list of bottleneck projects to be undertaken in as part of their 

congestion management activities. This process was accomplished in a span of 

three months from late January 2007 through mid-April 2007 (see Exhibit 5). The 

overriding strategy for this process was to identify smaller-scope, lower-cost projects that could be delivered within two years and would signifi cantly relieve congestion 

without pushing it further downstream. 
24
Empirical data is highly useful for both identifying a candidate pool of potential 

bottleneck locations as well as for ranking bottlenecks by the severity of the problems. Often this is a two step process:

1. Scan for potential bottlenecks using relatively simple methods. Most states 

have data systems capable of matching traffi c volumes with roadway capacity 

and these can be used to perform the initial scan. 

2. Perform more detailed analysis using travel time data or more sophisticated 

modeling methods. Here we want to produce objective estimates of congestion levels at each of the potential bottlenecks as well as to identify the root 

cause of the problems. Travel time data from detectors on urban freeways 

is now widely available through the activities of traffi c management centers. 

Exhibit 6 shows an example of how these data may be used to identify bottlenecks. Special travel time runs, aerial photography, or video of suspected 

bottleneck areas can also be used to pinpoint sources of operational defi ciencies. Finally, private vendors are now offering vehicle probe-derived travel 

time data that can be used for congestion analysis and bottleneck identifi cation on virtually all highways.

Exhibit 6. Using Freeway Detector for Bottleneck Analysis
26

yy y y j, 

impacts of potential bottleneck remediation on upstream and downstream c

tions. The analysis will justify action to correct bottlenecks, confi rm the ben

of bottleneck remediation, or check for hidden bottlenecks along a corridor. 

conducting bottleneck analysis, care should be taken to ensure that: 

• Improving traffi c fl ow at the bottleneck location doesn’t just transfer th

problem downstream. The existing bottleneck may be “metering” fl ow 

that a downstream section currently functions acceptably, but the incre

fl ow will cause it to become a new bottleneck.

• Future traffi c projections and planned system improvements are inclus

the analysis. Safety merits also should be strongly considered.

• “Hidden bottlenecks” are considered. Sometimes, the queue formed by 

dominant bottleneck masks other problems upstream of it. Improving 

dominant bottleneck may reveal these hidden locations. It is important 

into account the possibility of “hidden bottlenecks” during the analysis 

• Conditions not traditionally considered by models are accounted for. T

are several bottleneck conditions, such as certain types of geometrics a

abrupt changes in grade or curvature, that can’t be analyzed by current 

sis tools. Engineering judgment will need to be exercised to identify th

problems and possible solutions.

These methods were successfully used to identify bottlenecks in the I-95 Co

(Exhibit 7). 
The topic of Volume Ten of the Traffi c Analysis Toolbox is 

on Localized Bottleneck Congestion Analysis, focusing on 

what analysis tools are available, necessary, and productive 

for localized congestion remediation. This Federal publication (FHWA-HOP-09-042) discusses when, where, and how to 

study small, localized sections of a facility (e.g., on/off-ramps, 

merges, lane drops, intersections, weave, etc.) in cost-effective 

means. Some chokepoints are obvious in their solution; add a 

turn lane, widen a stretch of highway, retime a signal, or separate a movement by adding a ramp. However, the solution can 

often lead to hidden or supplementary problems; hidden bottlenecks, disruptions upstream, or undue infl uence on abutting 

accesses, etc. Analyzing localized sections of highway is different from analyzing entire corridors or regions. This document 

provides the guidance that specifi es the choice of analysis tools 

and inputs necessary to analyze localized problem areas. 

This document provides the guidance that specifi es the choice of analysis tools and inputs necessary to analyze localized problem ar
28

A study for the I-95 Corridor Coalition used private vendor travel time data from INRIX, combined with ag

traffi c counts, to conduct an analysis of major bottlenecks along the corridor. The study used the data in th

lowing way:

• Scan INRIX data for potential bottlenecks.

♦ Speeds < 40 mph for time slice of interest for all of 2009.

• Combine adjacent links.

• Map and identify the physical features that are bottlenecks.

♦ Interchanges (mainly freeway-to-freeway);

♦ Bridges; and

♦ Toll facilities.

• Merge in volumes; compute delay and other performance measures (reliability and queue length).

• Estimate effect of bottlenecks on long distance trips.

© 2010 Google Earth © 2010 Europa Technologies
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 defi ciencies, 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 effi cacy 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 fi rst-and-foremost at the discussion of this strategy. This involves using a short section of traffi c-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 modifi cations. 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 modifi cations (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 modifi cations. 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 
30

through congested corridors, thereby delaying the onset of a brea

traffi c conditions. The result is decreased headways and more uni

behavior, which indirectly benefi t bottlenecks and chokepoints. 

• Zippering or self-metering that promotes fair and smooth me

motorist who is 10th in line knows that he will be 20th to merge int

lane ahead. This helps to eliminate line jumpers that bull ahead, d

queues, and often block adjacent lanes until they force their way in 

ally this method of merging requires on-site enforcement, but ofte

ited by regulars who know the process and are willing to abide. 

• Improve traffi c signal timing on arterials. Also, traffi c signal t

improvements at ramp terminal intersections will prevent ramp qu

backing up onto freeway main lanes. 

• Access management principles to reduce vehicular confl icts (he

on arterial corridors 

• Roundabouts. Roundabouts may be used in place of stop sign or s

trolled intersections, including replacing signalized intersections at ra

• Innovative intersection and intersection designs. A variety of 

designs are being implemented around the country (see below). 

• High-Occupancy Vehicle (HOV) or reversible lanes. 

• Provide traveler information on traffi c diversions. 

• Implement congestion pricing. Congestion pricing entails char

or tolls for road use that vary by level of vehicle demand on the fa

objective is to bring supply and demand into alignment. 

Innovative Intersection 

and Interchange Design Treatments1

In the past several years, several nontraditional designs have been devel

signalized intersections and interchanges. The alternative designs for in

all attempt to remove one or more of the conventional left-turn movem

the major intersection. By removing one or more of the critical confl ic

maneuvers from the major intersection, fewer signal phases are required 

operation. This can result in shorter signal cycle lengths, shorter delays

capacities compared to conventional intersections. Exhibits 8 and 9 sho

ples for two of these innovative designs.

1 Additional information on innovative intersection and interchange design treatments may be 

http://www.fhwa.dot.gov/publications/research/safety/09060/.
One such intersection design is the Continuous Flow Intersection, which eliminates one or more left-turn con

main intersection. This is achieved through dedicated left-turn bays located several hundred feet prior to the m

section, which allow left-turning vehicles to move at the same time as through traffi c. The left-turn traffi c signa

eliminated, allowing more vehicles to move through the main intersection and thus reducing traffi c congestion an

These at-grade intersections achieve traffi c fl ow similar to grade-separated interchanges, but at a considerably lo

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.
32

The double crossover diamond (DCD) interchange, also known as a diverging diamond interchange (DDI), i

interchange design that has much in common with the design of a conventional diamond interchange. Th

difference between a DCD interchange and a conventional diamond interchange is in the way left and through 

ments navigate between the cross street intersections with ramp. The DCD design accommodates left-turning 

ments onto arterials and limited access highways while eliminating the need for a left-turn signal phase at sig

ramp terminal intersections. On the cross street, the traffi c moves to the left side of the roadway between the 

ized ramp intersections. This allows drivers of vehicles on the cross street who want to turn left onto the ram

chance to continue to the ramps without confl icting with opposing through traffi c and without stopping. 
Are Deploying LBR Treatments 

and Developing Programs

Successful LBR Treatment Applications

Many transportation agencies have recognized that low-cost treatments can 

provide effective congestion relief at bottlenecks. A wide variety of improvements have been implemented and many innovative improvements are emerging. 

The following section provides expanded explanations of how these transportation agencies used strategies to improve congestion at bottlenecks. Exhibits 10 

through 14 present summaries of these successful LBR treatments. Exhibit 15 

highlights how VDOT’s STARS program approaches the LBR problem.

Exhibit 10. Successful LBR Treatments 

Improvements: Austin, Texas U.S. 183
34

Improvements: Arvada, Colorado

Exhibit 12. Successful LBR Treatme

Improvements: Saginaw, Michigan
Improvements: Metroplan MPO/Little Rock Region

Exhibit 14. Successful LBR Treatment

Improvements: Pittsburgh, Pennsylvania
36

In 2007, the Virginia Department of Transportation (VDOT) developed the STARS (Strategically Targeted A

Roadway Solutions) program. VDOT noticed that during the course of conducting screening analysis for crash 

for its Highway Safety Improvement Program (HSIP), many locations also had a congestion or bottleneck proble

decided that in addition to safety, mobility problems should also be included in the screening process.

Virginia’s STARS Program Spurs Rhode Island to Develop Its Own STARS Program

Learning of interest from Rhode Island DOT (RIDOT), FHWA facilitated a peer exchange with VDOT. This led 

developing a companion program, RISTARS. By identifying both safety and mobility problems simultaneously, 

would otherwise be conducted separately are combined. Further, it is often true that fi xing safety problems hav

benefi t for mobility, and vice versa. 

Identifying Study Locations

Step 7  Refi ne High-Priority Corridors Identify Safety and Congestion Hot Spots

• Conduct preliminary safety and congestion asse

• Field inspection

• Analyze crash trends

• Utilize internal tools 

(Integrator, ROW images, 1/4 mile crash densit
Unless transportation agencies make low-cost bottleneck improvements an explicit 

presence, it is likely that they will be overlooked or delayed; either deemed part of 

a “larger” problem, or unnecessarily postponed to some indefi nite out year. There 

are many ways to combat this: 

• Create a unique bottleneck program area. By developing an annual 

“named” program, agencies can effectively identify, fund, and most importantly, advance low-cost treatments. A stand-alone program also has the 

added benefi t of demonstrating to the public that the agency is actively 

engaged in fi ghting congestion.

• Undertake occasional “special projects” to focus on bottlenecks. Lowcost bottlenecks can be addressed through occasional “special projects.” For 

example, the Minnesota DOT conducted a “one-time” special compilation of 

projects meeting certain candidacy requirements. In much less than one year, 

MnDOT developed a highly accelerated process for bottleneck identifi cation and prioritization, which led to many effective projects that were implemented in the following two years. 

• Integrate consideration of low-cost bottlenecks into existing programs. 

Low-cost bottlenecks can be addressed programmatically even without a special program. By making them part of ongoing planning and processes, the 

can be part of an agency’s congestion arsenal. 

The following provide comparisons of how different state agencies have incorporated low-cost bottleneck projects into their planning and programming processes:

• The California Department of Transportation (Caltrans) does not have a 

formal bottleneck planning process; rather, bottleneck issues are addressed at 

the district level as part of the regional planning process. Much of Caltrans’ 

operational planning is guided by the Transportation Management System 

Master Plan, which sets forth the types of strategies that should be pursued 

in improving congestion. In much of California’s metropolitan areas, traffi c 

congestion is a 24/7 occurrence, and traffi c management is a 24/7 job. Bottlenecks are tweaked “in real-time” as part of their Corridor System Management 

Plans (CSMP), which are developed for some of California’s most congested 

transportation corridors. System monitoring and evaluation is seen as the 

foundation for the entire process because it cannot only identify congestion 

problems, but also be used to evaluate and prioritize competing investments. 

Caltrans does not have a direct funding for bottlenecks, although bottleneck 

projects are routinely programmed through the CSMP process.
38

didate projects and drives the statewide highway project selection and 

process. Beginning in 2002, ODOT developed a “congestion mappi

that uses volume/cost (V/C) ratios developed from traffi c data recor

roadway inventory. About the same time, ODOT administration p

an annual process of overlaying congestion index and safety index 

As a result, congestion hot spots now have a “voice” in the proces

of crash indices, and congestion-related problems now compete fo

in the HSP listing. Specifi cally, highway sections with V/C ratios g

1.0 are considered “congested” and are added to the listing. Sectio

V/C between 0.9 and 1.0, but outside the cities of Columbus, Cinc

Cleveland, are also added. After ODOT headquarters completes t

wide effort of congestion mapping and safety indexing, the respec

engineers are responsible for developing countermeasures for their 

candidate projects. District Safety Review Teams sort projects into 

– low (less than $100K and quickly implementable), medium ($100

and one to two years), and high (greater than $5M and necessitatin

two years to implement) – and then compete with other projects h

same scale but in other districts.

• Minnesota DOT (MnDOT) was originally driven to explore low

congestion relief projects because of budgetary restrictions, bu

realized that these projects could be implemented very quickly a

bonus, were highly visible and popular with the public. In much 

one year, MnDOT developed a highly accelerated process for b

identifi cation and prioritization, which led to many effective pro

the following two years. MnDOT also found that because of lo

it could identify multiple locations throughout the region and “

around” bottleneck reduction projects in a fair and equitable ma

process consisted of completing a study, which included a fi ve-

cess to narrow potential projects into a recommendation list to 

legislature. Evaluation of completed projects produced high be

ratios, usually greater than 8:1. 

• The Maryland State Highway Administration (SHA) has a ded

gram of about $5 million per year for the identifi cation and implem

of low-cost traffi c congestion improvements at intersections. The 

genesis tracks to when SHA asked “what can be done if and when 

ject’s ‘no-build’ alternative is chosen?” The program has been well 

the public and local governments. Projects typically include low-co

that can be implemented quickly, such as signal timing upgrades an
areas that have reduced congestion bottlenecks at a low cost.

• In Florida, there is not a “bottleneck” planning process, per se; rather, 

bottleneck-related issues are addressed as part of the Florida Department of 

Transportation’s (FDOT) standard planning process. The planning process, 

which is managed by the FDOT Systems Planning Offi ce, begins with needs 

identifi cation conducted at the district level, then projects are developed and 

proposed for the Cost Feasible Plan. The Cost Feasible Plan is adopted and 

projects are ranked for inclusion into the 5-year or 10-year programs. Traffi c data and the statewide model are used to identify defi ciencies, but it is the 

responsibility of the districts to identify and resolve hot spots.

• Washington State DOT has no direct funding for bottlenecks, but formally recognizes “bottlenecks and chokepoints” in their project planning 

and development process and devotes a portion of the Washington Transportation Plan (WTP) to them. At the planning stage, WSDOT considers 

bottlenecks together with traditional corridor improvements in a category 

called “Congestion Relief ” – bottlenecks do not have their own category for 

assessment or funding. The Congestion Relief projects are ranked (prioritized) using the benefi t/cost ratio and other qualitative factors.

• Additionally, the “Moving Washington” initiative, a special 10-year program, 

specifi cally recognizes the importance of the short-term low-cost improvements that are the hallmark of LBR projects. In “Moving Washington,” Tier 

1 projects are “immediate, low-cost, operational fi xes.” Another aspect of 

“Moving Washington” relevant for LBR programs is its reliance on performance measurement – not just to identify problems but to assess to the 

impacts of completed projects. More information on the use of performance 

measurement by WSDOT may be found in their “Gray Notebook”: http://

www.wsdot.wa.gov/accountability/GrayNotebook.pdf.

Want More Information?

The LBR Program is just one of several program areas dealing with congestion 

problems. More information may be found at FHWA’s “Focus on Congestion” 

web page at: http://www.fhwa.dot.gov/congestion/links.htm. 

The LBR Program has a comprehensive web site with additional information 

(http://www.ops.fhwa.dot.gov/bn/lbr.htm) and resources (http://www.ops.fhwa.

dot.gov/bn/index.htm).
40

Auxiliary lanes – Typically, any lane whose primary function is not sim

through traffi c. This can range from turn lanes, ramps, and other single 

lanes, or it can be broadened to imply that a traffi c bearing shoulder can 

in peak periods to help alleviate a bottleneck, and then “shut back off ” 

peak is over.

Bottleneck – There can be many defi nitions. Here are a few that are ty

1) A critical point of traffi c congestion evidenced by queues upstream an

fl owing traffi c downstream; 2) A location on a highway where there is lo

cal capacity, surges in demand (traffi c volumes), or both; 3) A point whe

demand exceeds the normal capacity; and 4) A location where demand f

a highway section periodically exceeds the section’s physical ability to han

is independent of traffi c-disrupting events that can occur on the roadwa

Capacity – The maximum amount of traffi c capable of being handled 

highway section. Traffi c engineers usually speak in terms of “free fl ow” 

Congestion (specifi cally, traffi c congestion) – FHWA’s Traffi c Cong

Reliability Report defi nes congestion as “an excess of vehicles on a por

roadway at a particular time resulting in speeds that are slower – someti

slower – than normal or free fl ow speeds. (Congestion is) stop-and-go 

Previous work has shown that congestion is the result of seven root cau

interacting with one another.” Since a bottleneck is a cause of congesti

tion cannot be solely analogous to a bottleneck. Congestion is more. F

a “congested” corridor may harbor multiple bottlenecks or any combin

seven root causes of congestion.

Downstream traffi c – Traffi c that is beyond (past) the subject point on 

Hidden bottleneck – A highway location where some type of physical 

is present, but traffi c fl ow into this area is metered by an upstream bottle

location does not appear as a bottleneck under prevailing conditions. Re

the upstream bottleneck will cause the hidden one to emerge as a new bo

2 The seven root causes are physical bottlenecks (a.k.a. “capacity constraints”), traffi c incidents, 

weather, poorly timed signals et al., special events, and over-capacity demand (i.e., daily and 

superimposed on a system with a fi xed capacity). Some sources cite only six root causes beca

over-demand as an inherent sub-element necessary for any of the other causes to exist in the fi 

another way, absent over-demand, there would just be “volume,” but not necessarily “conges
Nonrecurring events – As it pertains to traffi c, a delay caused by an unforeseen event; 

usually a traffi c incident, the weather, a vehicle breakdown, a work zone, or other atypical event. Even if planned in many cases, like work zones and special events, they are 

irregular and not predictably habitual in location and duration. 

Ramp metering – The practice of managing access to a highway via use of control 

devices such as traffi c signals, signing, and gates to regulate the number of vehicles 

entering or leaving the freeway, in order to achieve operational objectives. The intent 

of ramp metering is to smooth the rate at which entering vehicles will compete with 

through vehicles. Done properly, ramp metering will calm the “mix” that occurs at 

these junctions.

Recurring event – As it pertains to traffi c, a recurring event is a traffi c condition (i.e., 

a bottleneck or backup) that one can presume to occur in the same location and at the 

same time daily, albeit for weekday or weekend conditions. Examples would be peakhour slowdowns at junction points, intersections, and ramps. One can “plan” for these 

events because one knows by routine that such events will occur time and again in the 

same manner and place.

Traffi c microsimulation tools – Complex microsimulation tools that rely on input of 

traffi c data, intersection “nodes,” facility “links,” and the associated parameters of each 

input, in order to output simulated conditions. By changing the inputs, engineers can 

test different sizes, characteristics, and out-year scenarios of traffi c demand.

Upstream traffi c – Traffi c that has not yet arrived at the subject point on a highway.
42

gy pp

and delays due to recurring operational conditions or nonrecurring traffi c-infl uencing events.

Occurrences Recurring: “Predictable” in cause, location, time of day, and approximate duration.

Nonrecurring: “Random” (in the colloquial sense) as to location and severity. Even if planned in some cases, like work zo

events, these occurrences are irregular and are not predictably habitual or recurring in location.

Causes Recurring: Operational Causes – A “facility determinate” condition wherein a fi xed condition (the design or function of 

at that point) allows surging traffi c confl uence to periodically overwhelm the roadway’s physical ability (i.e., capacity) to hand

resulting in predictable periods of delay.

Examples Recurring: Ramps, lane drops, weaves, merges, grades, underpasses, tunnels, narrow lanes, lack of shoulders, bridge lane re

curves, poorly operating traffi c signals.

Supplementary Terms 

(applies to either type) 

“Active” bottlenecks – When traffi c “released” past the bottleneck is not affected by a downstream restriction (i.e., queue 

another bottleneck. “Hidden” bottlenecks – When traffi c demand is metered by another upstream bottleneck(s); i.e., either 

nonexistent bottleneck that would increase or appear, respectively, if only unfettered 

Identifi cation of

(applies to either type)

Motorists typically refer to bottlenecks in terms of added time delay when compared to the same nondelayed trip, but engin

agencies also measure performance data: average speed (travel time), lane densities, queue lengths, queue discharge rates, ve

travel (VMT), and vehicle hours of travel (VHT). 

Measurement of 

(applies to either type)

Data is collected using manual techniques (e.g., fl oating cars, aerial photography, or manual counts from video recordings) o

surveillance (e.g., detectors, radar, video, etc.) collected in real time. Modeling, especially microsimulation, can be used to stu

of bottleneck remediation on upstream and downstream conditions.

Classifi cation of Recurring: Type I – Demand surge, no capacity reduction (typically at freeway on-ramp merges). Type II – Capacity reduc

demand surge (typically changes in freeway geometry; lane drop, grade, curve). Type III – Combined demand surge and cap

(typically in weaving sections).

Nonrecurring: Usually classifi ed by the type of event (e.g., incident, work zone) and severity of impact (e.g., duration of th

lanes lost, closed, or impassable).

Signature Trigger Recurring: Bottleneck is due to over-demand of volume (i.e., peak-hour conditions). The bottleneck clears from the rear o

volume declines.

Nonrecurring: Bottleneck is due to loss of capacity due to an incident, or short-term over-demand due to a spot event. T

clears from the front or rear of the queue, depending on whether the cause is incident-related (former) or volume-related (la

respectively.

Disappears when Recurring: When volume over-demand drops back to manageable levels for available capacity (i.e., when off-peak conditio

Nonrecurring: When dynamic event is removed; queue should dissipate, thereafter.
gy pp

and delays due to recurring operational conditions or nonrecurring traffi c-infl uencing events.

Practical Mitigations: Recurring: Corridor Congestion Recurring: Localized Bottleneck Nonrecurring

Dynamic pricing Use shoulder lane Improve incident response c

reduce incident impact; redu

time for clearing incidents; re

“downtime” during the even

In work zones, maintain max

of open lanes during peak tim

durations using innovative m

contracting practices; minimi

times a section is an active w

combining improvements (e.

safety) and using highly dura

employ least intrusive detour

Pre-plan for and coordinate 

to adequately and effi ciently 

traffi c, including not only the 

but the subordinate deliverie

emergency response, and pre

event activities.

Have predetermined detour 

particular sections of highwa

of weather- or incident-relat

including available tools (i.e., 

stands, VMS boards, public i

conduits, etc.).

Transit alternatives Restripe weave area

Ridesharing, telecommuting Improve merge area

High-occupancy lanes Widen, extend, remove, 

or consolidate ramps

Successive ramp metering Individual metered or signalized ramp

New construction Improve signalization or intersection design

Install frontage roads Install frontage road

Traffi c demand management (TDM) 

techniques

Effect “speed harmonization” as in Europe

Build park-and-ride lots Encourage “zippering”

“Downtown“ or cordon\congestion pricing Use access management techniques

Provide traveler information Provide traveler information

Proactive signal timing plans 

(including adaptive control)
U.S. Department of Transportation

Federal Highway Administration

Offi ce of Operations (HOP)

1200 New Jersey Avenue, SE

Washington, DC 20590

www.fhwa.dot.gov

FHWA-HOP-12-012

April 2012