June 2009


Understanding Bottlenecks ................................................................2

What is a “Traffic Bottleneck”? .........................................................................................................2

What is “Congestion”? .......................................................................................................................2

So Who’s to Blame?............................................................................................................................4

The Frustration of Congestion..........................................................................................................6

Why Can’t it be Like This All the Time? (Uncongested, that is!) ................................................6

A “Bottleneck” May be “Congestion,” but Congestion is Not Always “Just a Bottleneck” ....6

What Elements Typically Exist to Define a “Bottleneck”? ............................................................7

What Options Exist to Combat Congestion (and by Extension, Bottlenecks)?..........................8

Why Do Recurring Bottlenecks Occur?...........................................................................................9

What is Stopping Us from Fixing Bottlenecks?............................................................................11

Common Myths about Bottlenecks................................................................................................12

A Rogue’s Gallery of Infamous Bottlenecks.................................................................................13

What is FHWA Doing? ....................................................................16

Recurring Congestion Program Strategies....................................................................................16

Nonrecurring Congestion Program Strategies ............................................................................16

The Localized Bottleneck Reduction Program ............................................................................17

Identifying and Assessing Bottlenecks ..........................................20

How Do Bottlenecks Disperse?........................................................22

Understanding Merging at Recurring Bottlenecks ......................23

The Difference in Merging for Recurring and Nonrecurring Conditions................................23

Which is Best? “Early” or “Late” Merging?..................................................................................23

What Instruction is Given to Motorists ........................................................................................24

Early Attempts to Direct Motorists to Merge ..............................................................................25

Studies: Post 2000 ............................................................................................................................25

Merging “At-Speed” versus Merging “At-Crawl” ......................................................................27

Merge Principles ................................................................................28

Principle #1: “Go Slow to Go Fast” ..............................................................................................28

Principle #2: Keep Sufficient Gaps................................................................................................28

Principle #3: Zippering ..................................................................................................................29

Principles Put Into Practice: Variable Speed Limits and Speed Harmonization....................30

So How Does One Merge Safely? ..................................................................................................31
Evaluating Bottleneck Improvement Effectiveness ............

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

Examples of How Agencies are Dealing with Bottlenecks 

Minnesota ............................................................................................................................

Connecticut..........................................................................................................................

New Hampshire..................................................................................................................

Utah ......................................................................................................................................

Los Angeles, California......................................................................................................

Planning and Programming Bottleneck Improvements ....

Want More Information? ..........................................................

Definitions ..................................................................................

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

ii
The delays arising from traffic congestion are an unavoidably frustrating fact of

life. Or are they – unavoidable, that is? Why must we accept to take thirty minutes to make what should be a fifteen minute drive? Speed is distance divided by

time; unless of course one has to pointlessly add time sitting in bumper to

bumper traffic! In that case, speed drifts into “delay,” which in turn drifts into

our lives. In bygone days one could expect a vehicle trip to be governed only by

the “straight line” distance and the performance of car and driver (with a necessary

nod to speed limits too). In today’s world, that same trip often factors “expectations” of congestion delay caused by others. Not us, mind you, but others who, if

they would only get out of our way, would free that trip back to the bygone era.

According to a February 2007 Harris Poll, just over one-third (37 percent) of

respondents cite traffic congestion as a serious problem in the community, while

one-quarter say traffic congestion is a serious problem that is not being addressed.

But please don’t tell the thousands of practicing traffic engineers, planners, and

road workers that congestion is not being addressed. It’s just that much like

weather forecasting, traffic management is a dynamic activity 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 and ever-evolving “fronts.” But there

are some conditions we can do something about given the right resolve.

Much of recurring congestion is due to physical bottlenecks – potentially correctible points on the highway system where traffic flow is restricted. While

many of the nation’s bottlenecks can only be addressed through costly major

construction projects, there is a significant opportunity for the application of

operational and low-cost infrastructure solutions to bring about relief at these

chokepoints. This document, Recurring Traffic Bottlenecks: A Primer –

Focus 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 Primer is intended to be a dynamic work-in-progress. As newer strategies and

discourse become available they will be uploaded to the FHWA’s “Bottleneck” web

site http://www.ops.fhwa.dot.gov/bn/index.htm and the Primer will be periodically updated. This Primer constitutes “Version 2.” The Primer is a key resource

for Federal Highway Administration’s Localized Bottleneck Reduction (LBR) Program, providing 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.

“Close to half of 

tion happens day 

the same time an

Source: http://www

congestion/describin
What is a “Traffic Bottleneck”?

Webster’s dictionary defines a “bottleneck” as: i) a narrow or obstructed 

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

the elemental characteristics of traffic bottlenecks exist in these descriptio

However, a road does not necessarily have to “narrow” for a bottleneck to 

(e.g., witness bottlenecks caused by a weave condition, sun glare, or a vertic

Traffic bottlenecks (hereafter, bottlenecks) have a myriad of causes and d

The most egregious ones tend to be freeway-to-freeway interchanges, but 

know that smaller, lesser chokepoints are frustrating too. Many of these 

points are “operationally influenced bottlenecks,” defined as localized sec

highway where traffic experiences reduced speeds and delays due to recur

ational conditions. The fact that many recurring locations are “facility de

nate” (i.e., the design condition contributes to the resulting backup) is bot

encouraging and discouraging. Facility design is a tangible feature that can 

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

Bottlenecks may be compared to constricted storm pipes that can carry o

much water – during floods the excess water just backs up, much the sam

fic does at bottleneck locations. Like the friction that results from turbule

pipe, once traffic flow breaks down to stop-and-go conditions, capacity is 

fewer cars can get through the bottleneck because of the extra turbulenc

options available to free up “the pipe” are to carry less water, increase th

the pipe or system, or remove whatever constriction exists (i.e., remove 

reduce cavitations, or reduce head loss due to friction at junctures, et al.

ously the pipe comparison is analogous to options available to improve 

flow; namely, to reduce demand (by effecting driver behavior), build new 

structure, or improve that which already exists. Bottleneck mitigation is 

of these options.

What is “Congestion”?

FHWA’s Traffic Congestion and Reliability Report defines congestion as “

of vehicles on a roadway at a particular time resulting in speeds that are s

sometimes much slower – than normal or free flow speeds; [Congestion i

and-go traffic.” The root causes of congestion have long been understo

there is now broad consensus that congestion generally reflects a funda

imbalance of supply and demand. That is, during hours of peak usage o

transportation facilities most desirable to motorists, the supply of roadwa

is insufficient to meet the demand for those facilities.

2
devise strategies that will either mitigate or reduce congestion.

Recurring congestion happens in roughly the same time and place on the same

days of the week. It results when physical capacity is simply not adequate to accommodate demand during peak periods. Causes of recurring congestion include inadequate physical capacity and poor signal timing.

Nonrecurring congestion occurs randomly and is caused by events such as work

zones, traffic incidents, and bad weather. Even if “planned” in some cases, these

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

Obviously, when these nonrecurring events occur on an already congested facility,

the impacts are magnified. Figure 1 presents a pie chart showing the factors that

cause traffic congestion.

Inadequate

Physical

Capacity

(Bottlenecks)

40%

Traffic Incidents

25%

Work Zones

10%

Bad Weather

15%

Poor Signal Timing

5%

Special Events/Other

5%

Figure 1. Sources of Traffic Congestion

Source: http://www.fhwa.dot.gov/congestion/describing_problem.htm.
other side says, “There are too many cars!” And the edge of the coin rea

“What’s out there doesn’t work well!” Boy, it’s just like humans to blame 

thing else – except ourselves.

The title of Tom Vanderbilt’s 2008 book “Traffic: Why We Drive the Wa

(And What it Says About Us)” reflects the human traits – and not the phy

work – that have conspired to result in the system that we have (and how 

it). Sure, there is the inevitable “state of the system” baseline that discuss

of the network, some measures of effectiveness (or lack thereof) and a “w

we go from here” look forward. But an equal amount of pages are given 

chologists as to engineers in explaining why the need to overcome our hu

drives the incremental technological advancements; something akin to the 

adage that building better and more roads begets buying more vehicles an

more trips. These human factors include driver attention, reaction, and te

and explain that we are creatures of habit or need (witness our penchant 

the peak hour despite our hatred for it), that we essentially drive “for our 

gain” instead of for the greater good (e.g., reference his comparisons of a

other animal colonies that seem to work for a common goal), and that we 

myriad of transformations (e.g., road rage, selfishness, vehicle envy, distra

gadget, single-driver preference, et al.) that conspire to erode our driving 

civility, the latter of which we are otherwise forced to address face-to-face 

are outside of our four-wheeled cocoons. Inside our cars we become em

protected by 2,000 pounds of steel and anonymous behind our tinted win

Any slight – in terms of delay, encroachment in our lane, or imposition o

ceived “right” to an unencumbered trip – is a personal affront. A driver p

we feel compelled to pass him. We switch to the “fast lane” and invariabl

others, such that we are now in the “slow lane.” Vanderbilt cites research 

demonstrates 1) how this creates “density friction” that degrades efficienc

fic for everyone; 2) how you and a neighboring “spotter” car will progres

equal; and 3) the fact that all that jockeying gains nary a time advantage w

weighed against the nominal case of just staying with one lane. But still w

jump. Compare to the fact that if one flips a coin enough times, the od

heads versus tails eventually narrows to 50 percent each. But we contin

switch lanes – or play the odds – thinking that we can beat them, becau

once in a while, we do. Further evidence of our human consequence exi

fact that most all agencies now use the term “crashes” and not “accidents

record those incidents, to overcome the excuse that “accident” somehow 

4

George

Carlin famously

observed that

“everyone driving

slower than you is an

idiot and everyone

faster is 

a maniac.”
awake, not talking on your cell phone, or not driving while impaired. 

Specific to the topic of congestion (and bottlenecks), Vanderbilt’s citations confirm what we intrinsically know; that in survey after survey the majority of us

consider ourselves superior drivers to practically anyone else despite that in other

surveys we are very poor at estimating time spent in delay, distances traveled, and

the cost of that delay. A quarter is worth twenty-five cents to everyone, but a

minute is overestimated by the guy sitting in a queue, and underestimated by the

guy racing to an appointment. 

The aforementioned “superiority” manifests itself in our belief that somehow we

are more worthy of being on the road and it is everyone else who is clogging it up.

We are ridiculously inept at estimating our own speed (“I was going 30 MPH”

when in fact he was clocked at 46); estimating time spent circling for a parking

space (“I must have spent 5 minutes looking” when in fact she only spent 1 minute

45 seconds from street to space); and estimating others’ speed (“everyone travels

50 down my street” when in fact, a radar study found an average of 34 MPH and

only two cars doing above 46 MPH in a three-day study). Or maybe in the latter

case, it was the same car observed twice; hard to tell, but a moot point in terms of

accusing “everyone.” We often travel at a speed that the road “allows” and not

what the sign mandates. We’re not intentionally flouting the law, but hey, we know

better than the police and the engineers what it takes to get from here to there!

After all, the engineers dutifully allow for a “safety factor” in their design; that’s for

me, right? Sometimes the prior examples of estimation can be written off as exaggeration but research consistently reproduces like results when people are observed

first hand, or when focus groups are asked situational questions. 

In the context of “a road never jumped up and bit me” it seems hardly right to

“blame” the roads, absent poor maintenance or a malfunctioning signal. So we

blame the planners, the engineers, the politicians, and the “idiots” on the road.

Never mind that those same “idiots” are the neighbors, customers, and peers we

somehow otherwise abide. Clearly our desire to drive more often, and further, and

own more vehicles, has strained our major highway arteries. The effort becomes to

reduce the rate of congestion, if not to eliminate it altogether in as many locations

as funding and right-of-way can bear. All the while we will swim upstream against

perpetual waves of drivers, mileage, and demand. So the battle against congestion

wages on while our human instincts contribute to the malaise. 
6

frustrated because we feel we can’t control it. We can control most every

in our lives. We control what, when, and how much TV we watch becaus

the remote and the DVR! We control what, when, and how much we eat

best of our ability, we control what we do (our jobs) and where we do it; 

we’d like a little more control on how much they pay us! We control our p

our hobbies, and our vacations. We somewhat control our spouses and ch

at least we have influence upon them. But unlike the scene in Bruce Alm

can’t part the “sea” of traffic in front of us like Jim Carrey did.

But do we have to accept congestion? Our grandparents – and theirs – d

to endure the levels of congestion that we see today. But they also didn’t 

extent of today’s roadway network, the availability of gas stations and ser

the efficiencies and perks of modern cars. Time marches on, not back, so 

left to suffer the ravages of our success.

Why Can’t it be Like This All the Time? (Uncongested, That I

Shopping mall planners design parking lots for the twelfth highest dema

the year so as not to “waste” valuable land that would otherwise be squa

96 percent of the time. (Similar to insurance actuary tables, the “12th h

axiom is an accord based on the intrinsic worth of land cost, parking de

and of course, retail profit.) No doubt, during the Christmas shopping 

we all would love to get those “unnecessary” spaces back! Recurring co

is sort of like that, except that highway builders don’t intentionally unde

just seems that way. Highways are built to “design hour” and “design ye

tenets, but population and traffic demand are ever growing – and rarely 

over time. Outside of peak hours, the great majority of highway facilities a

than sufficient for the dynamic demand. Unfortunately, we tend to see 

at their busiest hours. But we occasionally experience the off-hour cond

too, i.e., when traffic flows freely. And like those too-few vacations whe

pampered and coddled, we ask “why can’t it be like this all the time?” R

congestion is a function of physical constraints, as well as how and whe

interact with a given facility and other vehicles on the facility.

A “Bottleneck” May Be “Congestion,”

but Congestion is Not Always “Just a Bottleneck”

The word “paper” can mean either a single sheet or a ream; likewise, “con

can be a single bottleneck – or much more. A bottleneck is distinguished fr

gestion in that it occurs at a specific location, and not pervasively along the 
of a facility, corridor- or systemwide “congestion” will inevitably result. It is overarching in nature. But when only subordinate segments of that facility are burdened,

then operationally recurring bottlenecks are said to exist. In this context then, a

bottleneck certainly constitutes “congestion,” but congestion cannot be said to be

universally analogous to a “bottleneck.” 

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 inefficient traffic signalization. The good news is that all

these things are potentially correctible by remediation. The bad news is that there

are many, many candidate locations, and agencies are fiscally constrained on how

much they can do. All things being equal, a recurring bottleneck will disappear

once traffic demand has decreased to a point where the operational or geometric

deficiency is no longer a factor, while a nonrecurring bottleneck will disappear only

after the random event has been removed. A nonrecurring bottleneck may be further impacted by traffic volumes, but it is not caused by traffic demand. Conversely, only a physical improvement will relieve a recurring bottleneck.

What Elements Typically Exist to Define a “Bottleneck”?

The Localized Bottleneck Reduction Program focuses on operationally influenced locations; that is to say, those that have a fundamentally design-based

cause, resulting in recurring delays of generally predictable times and durations.

The root cause of traffic flow degradation at the subject point of a recurrent

constriction is almost always a correctable problem. The following conditions

either exist or help to identify a recurring bottleneck condition.

• A traffic queue upstream of the bottleneck, wherein speeds are below

free-flow conditions elsewhere on the facility. (Note: if speeds at all or

most-all of the facility are consistently and regularly lower than free-flow

speeds, then overarching congestion exists. This is congestion beyond a mere

point-specific bottleneck location.)

• A beginning point for a queue. There should be a definable point that

separates upstream and downstream conditions. The geometry of that point

is often coincidently the root cause of the operational deficiency.

• Free flow traffic conditions downstream of the bottleneck that have

returned to nominal or design conditions.
ertheless theoretically “correctable” by design, as opposed to, say, 

phous, random event.)

• Traffic volumes that exceed the capacity of the confluence 

process traffic. (Note: this applies to recurring events even mo

than nonrecurring.)

What Options Exist to Combat Congestion 

(and by Extension, Bottlenecks)?

Fixing operationally influenced deficiencies applies to the fourth of the fo

four strategies available to combat congestion.

• Bring supply and demand in alignment through congestion pr

Congestion pricing or peak-period pricing entails fees or tolls for ro

that varies by level of vehicle demand on the facility. As with mark

in other sectors, road pricing helps allocate limited supply – in this 

of available road space. With user charges assesses at the point of 

greater efficiency results through improved response to market forc

Charges are typically assessed electronically to eliminate delays asso

with manual toll collection facilities. Road-use charges that vary wi

level of vehicle demand provide incentives to shift some trips to of

times, less congested routes, or alternative modes; or to cause some 

value trips to be combined with other trips or simply to be eliminat

gestion pricing has several important objectives. First, it seeks to b

demand with available capacity, i.e., the supply of road space. Seco

seeks to fairly allocate the costs associated with operating, maintain

expanding the transportation system to meet growing demand. Th

seeks to improve operation of the highway system. A fourth objec

include revenue generation. 

• Provide alternatives as to how, when, where, and if to travel. T

of this strategy is to reduce the number of vehicles on a given road. 

take the form of promoting alternative commute options such as em

telecommuting options or making transit easier and more attractive t

Also of interest in managing demand are driver incentive programs 

example, promote ridesharing and off-peak use.

• Invest in new highway capacity. Add new construction on new a

to preserve or improve system performance. 

8
management techniques, removing operational deficiencies, and improving

response time and management of traffic disrupting events like work zones,

accidents, and special events. Provide real time information about the system so that travelers can make immediate decisions about when, where, and

how to travel, and transportation agencies can make real-time adjustments

to improve system operations.

Why Do Recurring Bottlenecks Occur? 

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

weave areas, lane drops, tollbooth areas, and traffic 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 sufficiently and anonymously; however, when the design itself becomes the constricting

factor in processing traffic demand, then an operationally influenced 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 sufficiently high design

standards but are simply overwhelmed by traffic demand. Whatever the root cause,

operationally influenced bottlenecks can occur at:

• A lane drop particularly mid-segment where one or more traffic lanes are

lost. These typically appear at bridge crossings and in work zones. The latter, however, is a nonrecurring event and is usually remedied when the work

zone is removed. Ideally, lane drops should be located at exit ramps where

there is a large volume of exiting traffic.

• A weaving area, where traffic must merge across one or more lanes to

access entry or exit ramps. Bottleneck conditions are worsened when

there are confusing or insufficient weaving lengths.

• Freeway on-ramps, which are merging areas where traffic from local

streets can join a freeway. Bottleneck conditions are worsened on freeway

on-ramps without auxiliary lanes, short acceleration ramps, or where there

are multiple on-ramps in close proximity.

• Freeway-to-freeway interchanges, which are special cases of on-ramps

where flow from one freeway is directed to another. These are typically the

most severe form of physical bottlenecks because of the high traffic volumes involved.
have a short ramp length, traffic signal deficiencies at the ramp t

intersection, or other conditions that may cause ramp queues to 

onto freeway mainlanes.

• Abrupt changes in highway alignment, which occur at sharp 

and hills and cause drivers to slow down either because of safety 

or because their vehicles cannot maintain speed on upgrades. Anoth

ple of this type of bottleneck is in work zones where lanes may be r

or “shifted” during construction. 

• Low clearance structures such as tunnels and underpasses. Dr

slow to use extra caution, or overload bypass routes. Even suffic

clearances may impart problems. In one renowned case, the mer

illusion of a bridge appearing lower than it really is causes driver

down, resulting in recurring bottleneck conditions. 

• Lane narrowing, caused by either narrow travel lanes or narrow 

existent shoulders.

• Intended interruptions to traffic flow are literally “traffic disru

on purpose” that are sometimes necessary in order to manage sy

flow. Traffic signals, freeway ramp meters, and tollbooths can all 

tribute to capacity loss.

Figure 2 lists various types of freeway bottleneck causes.

10
What is Stopping Us from Fixing Bottlenecks?

The knee jerk reaction might be “lack of money.” But that’s everyone’s first complaint about, well, most every problem! In visiting with many states to ascertain if

they have a bottleneck-specific program or similar that targets chokepoint congestion, we have found a sampling of reasons.

• A predisposition to execute major projects. Certainly, no one is faulting

an agency which is attentive to executing major transportation initiatives.

There is no shortage of “fronts” on which to fight the congestion battle. A

short list would include HOVs, tolling and pricing, transit alternatives,

ridesharing programs, and bridge rehabilitation. But the onerous processes

involved in many of these initiatives can squeeze out smaller programs.

Freeway Bottlenecks

Primarily Capacity-Related Primarily Demand-Related

Interchanges Mainline

Geometry

Surges at 

Specific Points

Sys

High 

Freeway/

Freeway

Freeway/

Surface 

Streets

Lane Drop Grades

Weaving

Sections

Short, Steep,

Multiple 

Acceleration Lanes

Closely

Spaced

Interchanges

Exit Ramp

Geometry

Deficient 

Ramp Signal

Short 

Ramp Length
either mandate or adopt a program. High-level administrators form 

tion for their agencies. Mid-level managers’ production reflects their p

and skills in executing those initiatives.

• Lack of a “named” program. Unless there is an identity, bottlene

diation is usually relegated to a few projects done “by rote,” e.g., as 

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

posed projects.

• A culture of historical practices. Example: an agency that dutif

cuted an annualized “safety” program looked only at crash rates in 

mining their annual top ten list of projects. After instituting a con

mapping process, they identified several significant stand-alone ch

that did not correlate with their high crash mapping. Thereafter, h

gestion hot spots competed with high accident hot spots on their 

top ten list of projects. 

Common Myths about Bottlenecks

“Bottlenecks are caused only by not enough lanes on an extended highway 

In the past, recurring congestion was thought to be an overarching or sys

problem (i.e., not enough lanes) that could only be resolved by widening a

corridor. However, the fact that other subordinate sections within the s

operate sufficiently demonstrates that the uniform highway segments m

necessarily be under-designed.

Traditional capital solutions often grew from the misconception that a m

facility should be designed to alleviate recurring congestion during the p

hours each day. The problem is that funding for such large scale projects 

ited, and right-of-way is often restricted, such that these projects take man

complete. As a result, recurring congestion historically goes untreated, or 

to compete against other worthy projects, until funding becomes available 

“catch up” to the problem that has grown from the day the facility opens

With a shift in focus away from the perception that recurring congestio

temic (and thus treatable with only large scale projects), it is possible to 

wider range of improvement strategies that are possible in the short-ter

these will never replace the need for corridor-wide fixes – especially at “

bottlenecks” such as major freeway-to-freeway interchanges – low-cost, 

turnaround bottleneck fixes can provide congestion relief at the point of o

12

Solving Recurring

Bottlenecks is a

Win-Win Situation

for Nonrecurring 

Incidents Too!

Strategies to alleviate bottlenecks will lessen delay

caused by nonrecurring

events such as accidents,

weather, work zones, etc.

For example, consider an

accident that blocks a single lane of traffic. If only

two lanes existed prior to

the accident occurring, the

impact would be greater

than if three lanes existed.

The capacity loss resulting

from the nonrecurring

event will be lessened due

to the improvements made

to the system to benefit the

recurring situation.
major capital projects, it has been assumed for many years that bottlenecks cannot

be fixed without massive reconstruction of an interchange or corridor. However,

there are numerous examples where agencies opted to make lower-cost improvements that resulted in significant improvements in traffic flow. 

“Improving a bottleneck won’t help traffic flow outside of peak periods.”

Because traffic-influencing events like incidents, bad weather, work zones, and special events can happen at any time, congestion is not restricted to peak times of the

day. Benefits realized from improvements made to address peak-period bottleneck

problems will invariably carry over to the times outside of the peak when congestion occurs.

A Rogue’s Gallery of Infamous Bottlenecks

On much of the urban highway system, there are specific points that are notorious

for causing congestion on a daily basis. These locations – which can be a single

interchange (usually freeway-to-freeway), a series of closely spaced interchanges,

or lane drops – are focal points for congestion in corridors. Major bottlenecks

tend to dominate the congestion problem in corridors where they exist, some even

acquiring nicknames from local motorists such as the “Spaghetti Bowl,” “Hillside

Strangler,” “Spaghetti Junction,” “Malfunction Junction,” and “Mixmaster”.

Over the past several years, transportation professionals have come to realize that

highway bottlenecks demand special attention. Several national studies have highlighted bottlenecks as a major congestion problem in urban areas. These studies

have raised the level of awareness about bottlenecks as a problem, warranting that

they be treated as a significant part of the congestion problem.

The American Highway Users Alliance (AHUA) conducted two studies of the nation’s

urban bottlenecks in 1999 and 2004. The studies produced rankings of the worst bottlenecks in terms of total delay to travelers and discussed what was being done to fix

the problems at locations where specific improvements had been scheduled. The

studies found that nearly all of the worst bottlenecks are major freeway-to-freeway

interchanges in large urban areas. The 2004 study updated the rankings and discussed three bottleneck improvement success stories – bottlenecks identified in

1999 that were subsequently improved or well under construction.
assessed. Bottlenecks outside of urban areas were also considered (e.g., s

grades). A major finding of this study was that in terms of total delay, th

bottlenecks – typically thought of as a commuter-related problem – are al

sources of truck delay.

States and regions are beginning to recognize the significance of bottlene

well. The Ohio Department of Transportation completed a study of frei

(trucking) bottlenecks, and the Interstate-95 Corridor Coalition is underta

study of all potential bottlenecks in Coalition states. The Atlanta Regiona

mission has defined bottlenecks as a specific portion of their Congestion 

ment Process and is identifying regional and local bottlenecks in their netw

More recently, an effort by a private data provider, Inrix, also identified th

worst bottlenecks (Table 1). Whereas previous bottleneck identification e

were based on analytic procedures using traffic volumes and capacity data

approach uses data assembled by them from a variety of sources. As dire

time measurements become more common and better refined, the scienc

tleneck identification and performance will improve.

14

Improvements 

Are Possible

“Seven of 18 bottlenecks

identified in 1999 – 

including hotspots in

Houston, Albuquerque,

Denver, Boston, Los 

Angeles, and Washington,

D.C. – no longer appeared

on (subsequent) rankings

of the country’s worst

chokepoints (due to) major

reconstruction projects

completed or underway.”

American Highway 

Users Alliance, 2004
2008

Rank Area Road/Direction Segment/Interchange State Length

Hours

Congested

C

1 New York Cross Bronx Expressway/I-95 SB Bronx River Parkway/Exit 48 NY 0.36 94 

2 San 

Francisco I-580 WB Bellam Boulevard CA 0.38 65 

3 New York Cross Bronx Expressway/I-95 SB I-895/Sheridan Expressway/Exit 4A NY 0.55 93 

4 New York Cross Bronx Expressway/I-95 SB White Plains Road/Exit 5 NY 0.27 87 

5 New York Harlem River Drive SB 3rd Avenue NY 0.15 81 

6 New York Van Wyck Expressway/I-678 NB Liberty Avenue/Exit 4 NY 0.58 77 

7 Los Angeles Hollywood Freeway/U.S. 101 SB Vermont Avenue CA 0.64 77 

8 Chicago Dan Ryan Expressway/

I-90/I-94 SB

Canalport Avenue/

Cermak Road/Exit 53 IL 0.52 77 

9 New York Harlem River Drive SB 2nd Avenue/125th Street/Exit 19 NY 0.23 84 

10 Chicago Eisenhower Expressway/I-290 EB U.S. 12/U.S. 20/U.S. 45/Exit 17 IL 0.98 57 

11 New York Cross Bronx Expressway WB/

I-95 SB Westchester Avenue/Exit 5 NY 1.15 77 

12 Los Angeles Hollywood Freeway/U.S. 101 NB Los Angeles Street CA 0.09 76 

13 Los Angeles Hollywood Freeway/U.S. 101 NB Spring Street CA 0.14 85 

14 Los Angeles Harbor Freeway/I-110 NB Adams Boulevard CA 0.13 73 

15 New York George Washington Bridge EB/

I-95 NB Center Avenue NJ 0.14 68 

16 New York I-95 NB U.S. 1/U.S. 9/U.S. 46/Exit 72 NJ 0.42 66 

17 New York Harlem River Drive NB Lower Level Washington Bridge NY 0.09 74 

18 Los Angeles Hollywood Freeway/U.S. 101 NB Alameda Street CA 0.26 73 

19 Chicago Dan Ryan Expressway/

I-90/I-94 NB Ruble Street/Exit 52B IL 0.13 76 

20 Los Angeles Hollywood Freeway/U.S. 101 SB Melrose Avenue CA 0.31 68 

21 Los Angeles Harbor Freeway/I-110 NB I-10/I-100/Santa Monica Freeway CA 1.09 70 

22 New Haven I-91 SB I-95 CT 0.47 63 

23 New York Van Wyck Expressway/I-678 NB Hillside Avenue/Exit 6 NY 0.27 79 

24 New York Van Wyck Expressway/I-678 NB Atlantic Avenue/Exit 5 NY 0.47 75 

25 Chicago Dan Ryan Expressway/

I-90/I-94 NB 18th Street/Exit 52C IL 0.34 75 

 Source: “Inrix’s National Traffic Scorecard: 2008 Annual Report,” February 2009, http://scorecard.inrix.com/scorecard/request.asp.
With regards to congestion, the Federal Highway Administration (FHW

motes a number of efforts to help reduce congestion on the nation's hi

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

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

to mitigate the problems.

Recurring Congestion Program Strategies

• Tolling and Pricing. Value pricing entails fees or tolls for road us

vary by level of vehicle demand on the facility. Fees are typically ass

electronically to eliminate delays associated with manual toll collectio

• Public-Private Partnerships. “Public-private partnerships” (PPP

contractual agreements between a public agency and private sector 

that allow for greater private sector participation in the delivery of 

portation projects. FHWA is working with our partners in the pub

private sector to further investigate these promising partnerships.

• Real-Time Traveler Information. This is “decision-quality” info

that travelers can access, understand, and act on to choose the mos

mode and route to their final destination. Timely and detailed info

about traffic incidents, the weather, construction activities, transit a

cial events, all aid in improving travel time predictability, better cho

reduced congestion.

• Traffic Signal Timing. Signal timing should correspond to the c

traffic patterns. Often signals are initially timed, but not re-adjuste

traffic patterns change. This results in inefficiency and unnecessar

Goal: work with state and local agencies in congested metropolitan 

and encourage best practices for improved traffic signal timing.

Nonrecurring Congestion Program Strategies

• Traffic Incident Management. This utilizes a combination of p

safety functions and traffic management functions – it requires co

between various public agencies to reduce congestion by clearing t

crashes and removing stalled vehicles. FHWA is championing law

and practices that speed up the clearance of major and minor incid

create congestion.

16
safety and mobility impacts to reduce congestion during construction projects.

The Highways for Long-lasting, Innovative, Fast construction of Efficient

(also known as HfL) and safe pavements and bridges pilot program as outlined

in SAFETEA-LU is meant to accelerate the rate of adoption of innovations

and technologies, thereby improving safety and highway quality while reducing

congestion caused by construction.

The Localized Bottleneck Reduction Program

The Localized Bottleneck Reduction (LBR) Program promotes operational and

low-cost bottleneck mitigation strategies to improve mobility in the short term.

Managed by the Office of Operations, the program serves to bring attention to the

root causes, impacts, and potential solutions to traffic chokepoints that are recurring events; ones that are wholly the result of operational influences. 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 overviews the wide range of operational and low-cost

strategies available to reduce congestion at bottlenecks. 

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

in the annual planning and programming processes.

• Regional workshops for state and local agencies to learn and share information on localized bottleneck reduction strategies and how they can be incorporated into state and local planning processes. 

• Guidance documents are forthcoming that are aimed at agencies and personnel who have first responsibility to address bottleneck congestion locations. 

In the longer term, a “toolbox” will be developed. The toolbox will be a onestop source for guidance on low-cost operational and construction strategies

for improving bottlenecks. FHWA will also issue guiding principles and concepts common to low-cost operational improvement programs in order to further state efforts in adopting a comprehensive approach to addressing traffic

congestion at bottlenecks.

Localized Bot

Reduction Pr

The FHWA Office of 

Localized Bottleneck 

(LBR) Program bring

the root causes, impa

potential solutions to 

chokepoints that can 

gated through relativ

improvements. The L

Program has several a

underway, including

• A Primer on local

neck reduction;

• A compendium of 

ated solutions to l

bottleneck problem

• Regional worksho

and local agencie

and share informa

More information is ava

FHWA Bottlenecks 

http://ops.fhwa.dot.gov/b
ized “hot spots” where the design of the roadway itself becomes the con

factor in processing traffic demand, resulting in recurring delays of gener

dictable times and durations. Mega-projects required to resolve major bo

problems and systemic congestion (e.g., entire corridor rebuilds, multim

additions, and systemwide improvements) are far and above the focus o

program area. Unfortunately, when weighed against these larger, more vi

projects, localized bottleneck problems often receive lower priority for fu

are put off entirely until they can be implemented as part of the larger, al

passing project. However, in this day and age of fiscal constraints, with a

facing over-escalating costs and increasingly limited right-of-way, it is evid

“business as usual” in resolving congestion problems no longer applies. L

bottleneck mitigations have several advantages that can help agencies deal 

these developments:

• They address current problems and therefore have high visibil

Agencies are under increasing pressure to do something immediatel

congestion problems. Because low-cost bottleneck treatments are s

scale, they can be implemented quickly, so benefits start accruing im

• They are highly cost-effective and usually have positive safety 

Low-cost bottleneck treatments could mitigate or reduce crashes wi

weaving and merging areas, thereby increasing the cost effectiveness 

to safety merits.

• They will be required as transportation funding for mega-pr

becomes more constrained. Major reconstruction projects are 

tified as the only valid solutions to relieve congestion at the worst 

neck locations. However, the cost of executing such projects is u

enormous. Low-cost bottleneck improvements provide an effecti

stretch scarce resources.

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

tions can be implemented throughout a region, addressing more cor

than just a few large projects.

18
terms of disruptions during construction and final design.

• They are not necessarily just short-term fixes. For some low-cost treatments, congestion benefits will play out over many years, not just a few. In

fact, when combined with other forms of treatment (e.g., demand management and operations), they may be part of a long-term solution for a problem location or corridor.

• They may be considered part of major reconstruction projects to

address current problems. Some state DOTs have successfully incorporated low-cost bottleneck treatments within the context of larger, multiyear

reconstruction projects.

Sugges

Opportunit

Lower Cost S

Respondents to a FH

Division Office surv

that 71 percent of bo

occurred on freeway

percent interchange 

tlenecks. Responden

36 percent of bottlen

specific plans underw

improvement. Full i

or freeway reconstru

the most commonly p

solutions for resolvin

necks. However, at 

tion of these bottlen

be candidates for low

tions that can be im

in the short term to 

traffic flow. 

Recent discussions w

state partners have r

their position. Specif

ranking state DOT o

mid-level staffers eng

day-to-day operation

opined that there are 

dous” and “significa

to pursuing low-cost 

improvements. Thes

range from the dire

tions in delay, incre

throughput) to the i

(public confidence a

image-boosting effec
20

Bottleneck locations can be identified through direct observation by agen

sonnel, aerial photographs, or video surveillance data. Some metropolitan 

have even conducted public outreach efforts to solicit input from motoris

tleneck locations. Traffic analysis tools can systematically identify bottlen

tions by analyzing road segments for congestion or poor levels of service

Once bottleneck locations have been identified, the root cause and severity 

determined. Special travel time runs, aerial photography, or video of susp

tleneck areas can be used to pinpoint sources of operational deficiencies. 

ways equipped with detection technology, dynamic surveillance can be use

identify where and how often bottlenecks occur, and how severe they are. 

traffic data can be used to measure whether the problem is growing or rec

Sometimes, the operational cause of a bottleneck is evident, intuitive, or 

dotal. However, when multimile corridor congestion is prevalent, micro

tion modeling can assist in identifying, separating, and analyzing bottlen

dynamics within the corridor. 

Data collection to support bottleneck analysis should be sufficient to capt

duration and extent of congestion. Typically, 15-minute traffic volume co

all ramps and mainlanes for a four-hour peak period is adequate. Other d

be collected through travel time runs, video, or origin-destination studies. 

Bottleneck analysis is necessary to study not only the subject location, but 

impacts of potential bottleneck remediation on upstream and downstream 

tions. The analysis will justify action to correct bottlenecks, confirm the b

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

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

• Improving traffic flow at the bottleneck location doesn’t just transfe

problem downstream. The existing bottleneck may be “metering” f

that a downstream section currently functions acceptably, but the in

flow will cause it to become a new bottleneck. 

• Future traffic projections and planned system improvements are inc

the analysis. Safety merits also should be strongly considered. 
the dominant bottleneck may reveal these hidden locations. It is important to take into account the possibility of “hidden bottlenecks” during

the analysis stage.

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

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

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

analysis tools. Engineering judgment will need to be exercised to identify

those problems and possible solutions. 

Solicits P

Feedback 

Bottleneck Lo

in Arkan

Metroplan, the M

the Little Rock, A

region, conducted 

tion Bottleneck in 

2008, a public ou

effort designed to 

traffic bottlenecks 

auto, bike, and pe

safety issues thro

region. The prog

received 3,000 re

11⁄2 months, with 

submissions cons

the highest return

plan is currently 

classifying, and a

the feedback, and 

step will be to coo

with local jurisdi
Recurring congestion usually disappears when the crush volume drops ba

level that is manageable by the facility design. Nonrecurring congestion u

disappears when the dynamic event is removed. The rate at which merge

unite is strongly governed by the rate of dispersal at the constricted point 

“nozzle”) and thus is less easily influenced on its own merits. In layman’s 

hardly matters how well the two lanes “knit” together when there’s no pla

There is some evidence that in nonrecurring events, strong direction in th

flagmen or specific messages (e.g., “Take Turns,” “Merge Here,” “Stay in 

may improve the rate of dispersal marginally. Such deployments are know

“Dynamic Lane Merges” (explained further below) as they rely on some a

proactive instruction. Realistically, the great majority of nonrecurring cho

are comparatively “static”; that is, there are signs forewarning the merge, b

is no proactive management of the merge. The information supplied at r

chokepoints is even worse; barely much beyond a free for all. Finally, an

lesser degree, the rate of arrival of vehicles upstream joining the back of 

queue has some impact on the bottleneck.

22

Can Merge?”

Are you a “profiteering”

lane merger, who seeks

only your own personal

gain to advance, or are

you an “altruistic” driver

who yields to others for

the benefit of all? 

Whether you are an

“early” merger (upstream

of the point of confluence)

or “late” merger (at the

last possible moment) does

not brand you good or

bad, Republican or 

Democrat, or even rightminded or left-minded.

 Anecdotally, both sides

claim pre-eminence in

their preference. 

In the end, the prevailing

conditions (i.e., whether

the merge is executed at

free-flow speeds or at a

crawl; what guidance is

provided) and the generalization that we are each

left to our own devices as

to consider when it is best

to merge, conspire to make

this more a study in

human behavior, and less

a study in efficiency.
The Difference in Merging for Recurring and Nonrecurring Conditions

Merging in a recurring backup is different than in a nonrecurring one. Outside of

ramp metering, the former is essentially “cat herding” with implicit rules (often

local in culture or habit) at best. Typically, not much guidance is given. Drivers

just “suddenly” encounter taillights ahead, whereas, in a nonrecurring event, there

is more apt to be instruction in the form of orange cones, signs, flagmen, 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 (traffic cops) from chaos. 

One might argue “What’s the difference? I’m in bumper-to-bumper traffic regardless!” The great difference is the greater potential (in nonrecurring) for herding

those cats. Nonrecurring congestion is better suited for empirical study, i.e., characterized by observation and trial instead of theory. The “Dynamic Lane Merge”

(as we will see later) is a prime example of this. Conversely, recurring congestion is

best suited for (and pursued by) the academic and scholarly; focusing on the root

causes, the queuing theories, and the algorithmic “solutions” that might bend or

break the cycle of recurrence. However, in real life recurring situations, drivers are

mostly left to their own devices how and when to merge. Their tendencies may be

studied, but “Exit 1⁄2-Mile Ahead” doesn’t really help once you are in the backup.

And therein lies the fundamental point; a recurring merge relies on a driver’s

predilection, as well as the aforementioned culture. For comparison, at nonrecurring events, an identical dynamic traffic control plan can be developed and implemented most anywhere and drivers will more or less treat it the same.

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

Can a better recurring merge be developed? Merging takes place at-speed or

“at-crawl.” The former is most often associated with free flow 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 confluence). This creates a matrix of four

possible merge conditions; 1) at-speed “early;” 2) at-speed “late;” 3) at-crawl

“early;” and 4) at-crawl “late.” To further complicate things, guidance concerning

where, when, and how best to merge can vary from modest to no forewarnings

in recurring conditions to fully deployed Traffic Control Plans (TCPs) in nonrecurring conditions. Given that this primer is focused on the recurring bottleneck

genre, the purpose of this section was to research if early or late merging was

best for these noncontrolled situations; i.e., when no active TCP exists. 
Work zones and special events are nonrecurring (random or irregular) c

wherein, dynamic TCPs are instituted for the expressed purpose of mit

the planned event, and are almost universally mandated by the oversight 

Orange cones, signs, flashers, and dynamic messages are typically deploy

motorist is given forewarning and structured instruction how to negotia

event zone. However, in recurring situations (chronic and habitual back

there is minimal guidance. In the best case (e.g., a freeway lane drop) th

couple of yellow standard warning signs and some pavement markings, 

dutifully forewarn for free flow conditions, but are essentially superfluo

jammed conditions. At many other recurring congested locations (e.g., 

hour crowding onto ramps or through intersections, et al.) there is no g

short of a free for all. This is because engineers build for, and sign fo

unconstrained condition. The result is that in many recurring situation

are rarely, if ever, given instruction where and how to merge, as they wo

a nonrecurring situation. This is important in framing the subject quest

namely, in recurring, jammed conditions, is there academic evidence end

“early” or “late” merging? 

Research could really only uncover data-driven studies in nonrecurring sit

(wherein, motorists are more or less directed to follow signs in a work zon

event-driven scenario) and anecdotal discussions (in newspapers and the o

community) in the recurring genre. Admittedly, in all of these cases, it m

tle in the discussion whether the cause was a recurring or nonrecurring 

rather the fact that for whatever reason, jammed conditions existed and 

ensued. In the computer simulation community, driver behavior is a rand

able used simply to enable the merge algorithms to execute and deduce ra

charge, queue lengths and duration of the congestion. Again, the point b

in simulation, the queues merge – somehow – but no import is given whe

merge occurs. 

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

together at recurring chokepoints. Personal preference, impatience, frustr

speed-differentials, and other human and vehicular traits conspire to influ

safety and reduce efficiency. Altruistic drivers are unselfish and yield – in 

degrees – to proactive drivers, who seek only their own benefit to cut in li

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

gies used in the work zone studies. Specifically, in setting up “Dynamic W

24
community’s penchant towards this method over the “early merge” option for

stop-and-go conditions. One theme, however, remained constant. Regardless of

the amount of forewarning and direction given to motorists (e.g., “light” guidance

in recurring situations and “heavy” guidance in nonrecurring situations) personal

preference seemed to win the day. Absent absolute enforcement, motorists were

observed to – or opined to – merge when and how they preferred, with less regard

for any instruction. Anecdotal conversation and local opinion columns (e.g., “Traffic Doctor,” “Mr. Gridlock,” etc.) have occasionally debated the merits of merging

early (upstream) versus late (at point of constriction). The fact that the debate even

lingers gives strength to the notion that no consensus emerged from academic and

Internet research. In Tom Vanderbilt’s 2008 book “Traffic: Why We Drive the Way

We Do (And What it Says About Us)” the author goes so far as to introduce the entire book with the “early” versus “late” debate, and tells why and how he became a

convert to the late merge.

Early Attempts to Direct Motorists How to Merge

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

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

and deceleration lanes, etc. Of course, at that time, traffic was less congested on

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

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

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

on educating motorists how to merge efficiently, unless one considers a

“queue” or a “traffic stream” as an entity that can deduce instruction. Nevertheless, the academic community has essentially confirmed, via queuing theory

and micro simulation that the discharge rate after the merge governs congestion on the segment. In layman’s terms, there is a finite capacity of the single

lane downstream of the constriction. Very little of what happens upstream

can refute the laws of physics; that only one vehicle can occupy the discharge

space at a time; and in a jammed situation, the lead vehicle does so from essentially a crawl speed.

Studies: Post 2000

There have been studies and trials of Dynamic Lane Merge Systems (DLMS) or

similar, wherein detectors and variable message boards are designed “to utilize the

best aspects of the early and late merge strategies” (Minnesota DOT, 2004).
was conducted in nonrecurring situations; specifically, in work zones. Ge

once queues were detected as growing, a series of dynamic messages wou

that would direct motorists where and how to merge during the stop-and

tions. Typically the messages advise motorists to “Stay in Lane,” “Merge 

the end of the dropped lane), “Fill All Lanes,” and even “Take Turns” or 

As the queues clear, the messages would turn off, returning the decision o

merge upstream of the work zone back to the drivers. Arguably, the real 

tors driving these trials are safety and increasing throughput in work zones 

nonrecurring conditions); that is to say that none of the trials was necessar

motorists’ preferences during the relatively more benign recurring conditi

mere fact that in jammed conditions, all of these trials presumed to set up 

merge” scenario speaks to the engineering community’s penchant towards 

method over the “early merge” option for stop-and-go conditions. The c

seems also to provide endorsement; namely, that absent jammed conditio

merge is “relaxed” and early merging is “allowed.” Note that we don’t us

word “encouraged” for the latter condition; we merely note that the decis

how and when to merge is abdicated to the driver. The 2004 Minnesota

study/comparison of four different deployments (locations, conditions) y

promising success only in terms of queue reduction and driver behavior. 

anecdotal evidence in the study to support that drivers are less aggressive

ceive shorter travel times when the overall distance traveled under conges

ditions is reduced. However, all things being equal, the rates of dissipatio

downstream of the constriction don’t seem heavily influenced by “early” 

merging in stop-and-go conditions, as evidenced by the study’s finding th

vehicles per hour per lane downstream of the constriction seems a maxim

rate for any deployment – a number that not coincidently is the suggested 

mum flow rate listed in the Highway Capacity Manual for “short-term fre

work zones.” (The reader is cautioned that the conclusions herein are mer

observations of the Minnesota study authors and should not be considere

mount to a broad brush conclusion for all bottleneck conditions and facto

ther, these deployments, with their garish orange barrels, cones, and VMS 

scream “temporary condition,” and are expensive to deploy on an ad-hoc 

not the kind of thing that one could expect to be adopted as standard pra

26
done at the national level to educate drivers how to merge safely and efficiently, as

compared to other national education efforts promoting seat belt compliance,

school zone safety, traveler information, 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 first 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

traffic blogs and an Internet search finds strong sentiment from both camps as to

why they think their method is best. 

Merging “At-Speed” versus Merging “At-Crawl”

Under free flow conditions, the Highway Capacity Manual states that 65 mph freeways are capable of enabling up to 2,300 pcphpl – a number that can be compared

to the 1,600 pcphpl “work zone” capacity-restricted volume above. (The 2,300

number takes into account safe vehicle separations and optimum conditions.) This

roughly 30 percent reduction in capacity is attributable to the fact that in stop-andgo conditions, the inefficiencies of merging are magnified. Whether or not those

two adjacent lines merge early, late, or otherwise, they are still metered by the fact

that only one vehicle can proceed past the constriction at a time. As upstream

volumes dissipate, free flow speeds increase, and vehicles enjoy a “head start” of

momentum through the opening, allowing for slightly higher discharge rates that

would fall above 1,600 pcphpl but cap at 2,300. In layman’s terms, merging at-speed

upstream of a constriction slightly improves the discharge capability, but there is still

a finite time necessary for all vehicles to clear in one lane downstream. 
28

For

What it’s

Worth

Webster’s on-line dictionary

defines “merging” (using ‘traffic’ as the colloquial) as “to

blend or come together without abrupt change.”

(Emphasis added) 

Can we increase the efficiency of merging prior to the discharge point? I

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

that traffic is inherently safer when all vehicles are traveling at or near the 

speed. Think of an orderly progression on a crowded escalator. Everyon

cocooned because they are going the same speed. Now imagine the bum

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 traffic circles. 

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

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

entry in a 2006 contest to demonstrate the meaning of “throughput maxi

tion”) is the “rice” experiment. In the first case, dry rice is poured all at o

funnel. In the second case, the same amount is poured slowly. Repeated 

erally conclude about a one-third time savings to empty the funnel via the 

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

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

experiment teach us about traffic?

The densely packed rice (or traffic) in the first trial creates friction in 

sense and the practical sense, respectively. The denser the traffic, the 

the safety cushion around each driver, and the more cautious (i.e., slow

becomes. A classic “bell curve” diagram also serves to explain how traf

throughput reaches an apex up to the point where traffic friction and con

spire to begin a decline in the rate of throughput and speed. There exis

examples of validation of this principle at intersections (e.g., traffic sign

roundabouts, vehicle detection) that demonstrates that slowing or stopp

traffic benefits the aggregate flow, and is far better than the free-for-all 

In the bottleneck and corridor genres, we have ramp metering and spee

nization (see below), respectively, providing examples on freeways.

Principle #2: Keep Sufficient Gaps

Keeping sufficient (or ideally, the largest possible) gaps leads to uniform a

free(er) traffic flow. Gaps allow for small adjustments in braking, accelera

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

vehicles, which otherwise would react by slowing. Gap maintenance is ac

rote in high-occupancy vehicle (HOV) lanes or high-occupancy toll (HOT

by selective admittance in the former, and by dynamically shifting the pric
pose (GP) lanes. Both cases have the added (and intended) benefit of removing

vehicles and or person-trips from the GP lanes too; so all traffic streams win when

these practices are employed. Absent out-and-out violators who can muck up the

system, agencies can tweak the lane mandates to keep the systems running at optimum levels.

How does this apply to localized bottlenecks? Theoretically, the same “gapping”

principles would hold true in backups; to wit, leaving progressively larger gaps

would allow for progressively better progression. (Taken to the extreme, no

“bottleneck” would even exist!) The point is that in congested situations the constant brake-tapping in bumper-to-bumper traffic works to self-perpetuate the

problem. No one can get much momentum before he or she has to react to the

vehicle directly ahead or adjacent. The ripple effects are short, abrupt, and inefficient. The obvious problem with this is that human nature simply won’t allow

for the patience and orderliness to make this work. The second I create a sufficient gap between me and the car ahead, some “profiteering” lane jumper will fill

it. Which is a nice segue into the next principle; zippering.

Principle #3: Zippering

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

could easily be melded into our regular practice; namely, to take turns, or “zipper”

merge at the front of the line. The fairness – and simple visualization – of this

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

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

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

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

moment and taking turns to get through the chokepoint nozzle. One enterprising

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

messages promoting this method. But is this latter day Don Quixote merely tilting

at this traffic “windmill”? To be fair, “late” merging seems the slightly favored

method in web blog research, although as noted prior, there are unyielding stout

proponents of the early merge too. Let’s take a closer look. 
put, and the zipper order is L, R, L, R. etc., your neighbor to the left will b

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

him into the L line (and your fellow motorists in the R line all move up in 

sion) then you will become 11th, the person originally behind you (former

now becomes 12th, and your neighbor will have dropped to 13th. You wi

neighbor loses. But the real winner is the guy originally behind you; he’s g

two spots.

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

orderly scenario you would be 11th and your neighbor would be 12th to m

you cut in front of him as in the first scenario, the guy behind you (who m

to fill your space) becomes 11, you are 12, and your neighbor is slotted at 

lose. Your neighbor really loses (drops two slots) and the guy behind you 

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

Congratulations! In both scenarios, by “early merging” you have definitely 

the slot for the guy behind you! You may or may not have improved your 

either way you made your neighbor mad! Of course, in real life, the perm

are mind boggling when one considers multiple jockeying for position in 

queue by you and others, or the dynamic created by multiple lanes. You m

you are making progress one line jump at a time, but your move(s) may h

canceled by someone way up ahead of you; or the woman opposite him. 

on. It effectively becomes akin to playing the lottery – you are at the mer

ers’ actions equal to, or more so, than your own. And all for the perceived 

of getting a few spaces ahead through the nozzle. Of course, there’s ano

to look at it too; it’s all a wash when said and done. It’s hard to know. Al

have accomplished by line jumping is to elevate yours or others’ stress. B

sit, wait, take your turn, and put your radio on some calming music.

Principles Put Into Practice: 

Variable Speed Limits and Speed Harmonization

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

European concept of “speed harmonization” (nonwork zones) both int

“harmonize” traffic by regulating speeds. In the latter case, a series of o

gantries gradually adjust speeds through congested highway segments in 

flatten the sinusoidal effect of traffic speeds bouncing between open se

and interchanges. Speed harmonization is typically effected as the open 

approaches the denser central business district. A great expense is incur

30
required along the multi-kilometer corridor; a smart highway, as it were. At the

end of the day however, such a system is not transferable from facility to facility,

so you had better have good justification and need to install it where you did. But

that’s the bad news too; that such pervasive congestion exists in the first place.

What if the congestion disappears (e.g., a parallel facility opens up or a large traffic

generator goes away)? The agency is stuck with an expensive boondoggle. 

Specific to the question at hand, speed harmonization does not seek to control

driver behavior to switch lanes, or mandate how to merge, but rather only to control speeds. “Go slow” (harmonize) as a means to move more traffic than otherwise might have gotten by. As of 2009, this system has not been employed in the

United States, although there is intent to try it in the Seattle, Washington area as

part of a Federally funded demonstration project. 

So How Does One Merge Safely?

What methods work best for motorists to merge traffic efficiently in bottlenecked conditions? The answer, it seems, is largely – perhaps only – dependent

on the flow rate of the continuing lane and less-so the dropped lane next to it.

The debate between “early” or “late” merge seems more appropriate for human

behavioral studies of aggressiveness, impatience, self-indulgence, and simple

preference for one method over the other. However, there is general agreement

in the engineering community on which method works best for each of two scenarios. Early merges work best under “Scenario 1” (see following page) when

traffic is generally free-flow. Late merges seem to work best under “Scenario 2”

when traffic is stop-and-go.
32

Scenario 2: 

In heavy congestion (stop-and-go conditions) it is more efficient to 

fill both lanes and zipper-merge at the point of constriction

Benefits: Drivers experience less stress when they understand that each in-turn will have a

chance to get past the point of constriction. The zipper merge is orderly and fair.

Potential pitfalls: 1) Queue jumping. This can occur in a number of ways; a) by bypassing

cars in line, the “jumper” infuriates others because he is out of turn; or b) by having a second

or third vehicle try to fill a single gap, this frustrates altruistic motorists who now feel they are

being taken advantage of. 2) Lane blocking. This occurs when a motorist in the continuous

lane fails to fairly permit his neighbor to zipper ahead. 3) Line jumping back and forth, as i

recalls the old axiom “the other line is always moving fastest.” This only serves to disrupt th

continuity of the merge ahead.

Bottom line: Remain orderly and fair. Do not line jump.

In atspeed, or nearly freeflow conditions, 

it is best to merge upstream of the constriction

Motorists in the thru-lane: Allow gaps. By allowing gaps you allow the merging vehicle the

opportunity to adjust his/her speed to fill a gap.

Motorists in the dropped lane: Keep up speed and fill gaps at-speed. Use the dropped lan

as an acceleration lane; i.e., as an opportunity to match speed with upstream traffic and merge

at-speed at earliest and best opportunity.

Benefits: Traffic is safest when all vehicles travel at or near the same speed.

Potential pitfalls: 1) When a vehicle in the dropped lane can’t or won’t move into the throug

lane in time, then the dropped lane ends, forcing the vehicle to suddenly slow or stop. This w

invariably cause a dangerous ripple as the stopped vehicle now must join moving traffic from

nearly a dead stop; at risk of causing a through vehicle to slow to allow it in. 2) Pitfall #1 abo

also has the potential to cause following vehicles to queue behind the “stopped” vehicle due t

an increasingly shortening merge lane. Motorists now forced to slow in the dropped lane beco

anxious to merge and may cause further ripples. If not allowed to remediate itself, pitfall #

will morph into “Scenario 2” below. 3) Motorists in the thru lane won’t yield (usually by wa

of speeding up).

Bottom line: Relax. Allow motorists to merge at-speed or fill gaps as opportunity allows.
Here is a sampling of short-term, low-cost operational and geometric improvements. All of these remedies address operational deficiencies, as opposed to other

congestion mitigation efforts that address driver choice, travel demand, corridorwide upgrades, or simply (but expensively) building our way out of congestion.

1. Shoulder conversions. This involves using a short section of traffic bearing

shoulder as an additional travel lane. Shoulder conversions are appropriate

between interchanges or to provide lane congruency with adjacent sections.

The shoulder condition be rated for use as a travel lane.

2. Re-striping merge or diverge areas to provide additional lanes, provide an

acceleration/deceleration lane, extend the merge/diverge area, or improve

geometrics to better serve demand. 

3. Lane width reductions. This involves reducing lane widths and re-striping

to add an additional travel and/or auxiliary lane.

4. Modify weaving areas by adding collector/distributor or through lanes.

5. Ramp modifications. These could include ramp metering; widening,

extending, closing, or consolidating ramps; or reversing entrance and exit

ramps to improve operations. 

6. Speed harmonization (variable speed limits). This is the practice of

adjusting speed limits when congestion thresholds have been exceeded and

congestion and queue forming is imminent. Speed harmonization can also

be used to promote safer driving during inclement weather conditions. This

mostly European practice reduces the traffic “shock wave” that results

through congested corridors, thereby delaying the onset of a breakdown in

traffic conditions. The result is decreased headways and more uniform driver

behavior, which indirectly benefit bottlenecks and chokepoints. 

7. “Zippering” or self-metering that promotes fair and smooth merges. A

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

single lane ahead. This helps to eliminate line jumpers that bull ahead, disrupt the queues, and often block adjacent lanes until they force their way in

line. Usually this method of merging requires on-site enforcement, but often

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

The bottl

solution, it se

not lie with the 

of merging, b

in reducin

underlying 

in the first 
34

onto freeway mainlanes.

9. Access management principles to reduce vehicular conflicts (henc

on arterial corridors 

10. Continuous flow intersections. These are unconventional at-grad

sections which eliminate one or more left turn conflicts at a main in

tion. This is achieved through dedicated left turn bays located sever

hundred feet prior to the main intersection which allow left turning 

to move at the same time as through traffic. The left turn traffic sig

is eliminated, allowing more vehicles to move through the main inte

and thus reducing traffic congestion and delays. These at-grade inte

achieve traffic flow similar to grade-separated interchanges, but at a 

ably lower cost.

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

12. Provide traveler information on traffic diversions.

13. Implement congestion pricing. Congestion pricing entails charg

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

The objective is to bring supply and demand into alignment. As p

acceptance grows and legislative restrictions are relaxed, congestion 

will increasingly be viewed by transportation practitioners as a powe

relatively easy to implement strategy to address bottleneck congestio

In 2006, as part of the research conducted for National Cooperative Hi

Research Program Project 3-83 (“Low-Cost Improvements for Recurrin

way Bottlenecks”), a series of interviews was conducted with state and l

transportation agencies to assess the effectiveness of low-cost improvem

used at bottleneck locations within their jurisdictions. Table 2 was devel

from these responses. The results showed that agencies are using a wide 

strategies to improve bottlenecks, many of them low-cost improvements 

be implemented quickly. The most frequently used operational improvem

ramp metering, auxiliary lanes, and HOV lanes. 
• Is there an inside shoulder that would create a usable traffic lane for a short

section of freeway? 

• If a shoulder is considered for conversion, is there right-of-way (ROW) to

allow adding one back for part of the length of the project? 

• If there are bridges, are they wide enough to accommodate the extra lane

while allowing adequate clearance to barriers (2 feet) and an outside shoulder?

If not, are they short enough that a loss of shoulder as a breakdown lane

would not be critical (500 feet or less)? 

• If changes to an entrance or exit ramp or weaving area are considered, will

adjusting the position of ramp gores cause geometric problems which must

be resolved? 

• Are vertical clearance issues, grade-matching, and sight distance problems created? 

• If the bottleneck movement itself cannot be fixed reasonably, can the other

traffic which is affected by it be better accommodated? 

• Finally, will the improvement invite enough new traffic to cause immediate

breakdown again or is this truly the clearing up of a “kink” in the system,

without being a capacity addition which will overload some other part of

the facility?1

1 These options quoted directly from recent work by the Texas Transportation Institute:

Freeway Bottleneck Analysis Methodology.
Bottleneck

Types

Mitigation Measures

Right 

Shoulder 

Conversion

Left 

Shoulder

Conversion

Lane Width

Reduction

Auxillary

Lanes

CollectorDistributor

Road

ReStripping to

Add More

Narrow

LanesAll

Purpose

Lane 

(Concurrent

or 

Reversible)

HOV Lanes

(Concurrent 

or 

Reversible)

Ramp 

Metering

Temporary

Ramp 

Closures

T

In

Heavy On-Ramp 

Demand

Weaving Sections

Lane Drops

Tunnels and Bridges

Horizontal and 

Vertical Curves

Narrow Lanes and

Lateral Obstruction

Inadequate 

Accelerated and/or 

Decelerated Lanes

= good solution = may be helpful = not applicable

36
After implementation, it is often beneficial to conduct an after evaluation to gauge

the effectiveness of the bottleneck removal project. A conservative approach to

evaluating treatment effectiveness is to evaluate operational and safety benefits

achieved. Common evaluation methods include microsimulation modeling,

cost/benefit analysis, and crash data analysis using data collected before and after

project implementation. In addition, the following performance measures are often

used to assess the effectiveness of bottleneck improvement strategies: average

speed (travel time), lane density, queue lengths, queue discharge rates, vehicle miles

of travel (VMT), and vehicle hours of travel (delay). Additional insight could be

obtained from before and after opinion surveys of area drivers. These types of

evaluations are often not done, yet are important to quantify the benefits achieved

through bottleneck mitigation.

Potential Issues with Bottleneck Treatments

Because some bottleneck treatments involve innovative solutions that maximize

effectiveness with a minimum of new construction, they are occasionally at odds

with highway design standards. For example, the addition of slip ramp to a collector/distributor road or the use of a shoulder a through lane at selected locations may not strictly follow allowable design standards. Such deviations have

the potential to degrade safety if not properly implemented; the elimination of a

shoulder may led to more collisions with roadside features or may impede incident management activities. As it is FHWA’s intent to foster creative approaches

for low-cost bottleneck improvements, agencies should not see the design standard issue as insurmountable. Rather, they should fully assess the potential

safety impacts of strategies and devise ways of addressing them, if necessary.

For example, in the case of a shoulder-to-lane conversion, review of crash data,

and the specific roadway location (perhaps through a Roadway Safety Audit), it

may be determined that a barrier is required to keep vehicles off of the roadside.

It may also require a change in incident management policy that would allow

emergency vehicles to access incidents from the opposite direction. Finally agencies should be in contact with the FHWA Division offices throughout the

process as design review may be required, depending on circumstances.
projects that have not been previously identified in Statewide Transpor

Improvement Programs or MPO-generated Transportation Improvem

grams. Thus, they may not be part of those projects that have been ap

to deal with air quality issues in the region or state. Such occurrences m

dealt with on a case-by-case basis by agencies wishing to undertake bot

improvements. One point worth noting: if air quality conformity in a 

precludes or discourages major capital expansion (e.g., additional lane-m

the type of improvements in a localized bottleneck program clearly do 

in this category. 

Examples of How Agencies 

are Dealing with Bottlenecks

Many transportation agencies have recognized that low-cost treatments ca

vide effective congestion relief at bottlenecks. A wide variety of improve

have been implemented and many innovative improvements are emerging

following section provides expanded explanations of how these transport

agencies used strategies to improve congestion at bottlenecks. 

• Low cost bottleneck improvements in Minnesota;

• Connecticut DOT’s low-cost efforts to improve a freeway diverge 

• Expediting bottleneck improvements ahead of major reconstruction 

in New Hampshire; 

• Continuous flow intersection improves intersection operations in U

• Operation Bottleneck Relief in Los Angeles, California.

For a more complete listing of representative projects, or for additional b

ground on specific projects, please visit the FHWA “Bottleneck” web site 

http://www.ops.fhwa.dot.gov/bn/index.htm or contact the Localized Bo

Reduction Program manager. 

38
Benefits as a Mega Project, but at a Vastly Different Cost

In 2006, Minnesota DOT utilized low-cost strategies to resolve a bottleneck on State Highway 100 betw

36th Street and I-394 in St. Louis Park, a suburb west of Minneapolis. This section of highway had onl

lanes, while upstream and downstream sections of SH 100 had three lanes. A prior study found that SH 

in this area was congested for six hours out of every day, the longest duration of congestion than on an

other freeway in the Twin Cities metropolitan area. 

A full third lane was added in each direction by narrowing the left and right shoulders. In addition, a ne

diamond interchange was connected via collector-distributor roads, reducing access points on SH 100 f

seven to four. The improvements eliminated weaving movements that existed along this stretch of on- 

off-ramps past the city. 

The result yielded a (some say conservative!) 13:1 benefit/cost ratio; an increased throughput of 14,400 

cles during peak periods daily; and a reduction in backups from 5 to 6 miles previously (depending on d

tion) to one-quarter mile. Minnesota DOT received an outpouring of positive public reaction, includin

local newspaper’s “Public Project of the Year” award.

Remarkably, this $7.1M project accomplished the same results as a $138 million project on a parallel fre

facility, IH-494 from Highway 5 to Highway 55. Granted, the IH-494 design/build project was closer t

total facility rehabilitation with noise walls and other costs, but the effective elimination of long bottlen

queues was realized with the SH 100 project for a fraction of the cost.
40

pg y g

Congestion routinely existed on I-84 eastbound in Danbury in advance of the I-84/Route 7 split. The exist

lane configuration in the vicinity of the exit ramp consisted of the left lane as an exit-only lane, with the cen

and right lanes continuing as through lanes. Because significant traffic exited onto Route 7 northbound, thi

arrangement created weaving problems and bottlenecking, as all traffic bound for Route 7 had to maneuver 

the left lane to access the exit.

Connecticut DOT devised a low cost solution which involved re-striping the lanes within the diverge area 

better serve demand. The I-84 eastbound lanes were reconfigured to allow the center lane to function as 

option lane, where vehicles would have the option to exit or continue on I-84. It was determined that th

existing pavement width in the vicinity of the ramp gore would accommodate the option lane and wou

tie in with the current lane configuration on the ramp, which included two lanes. The project required

minor signing and striping revisions and some pavement modification. The bottleneck improvements 

be completed as part of a larger interchange improvement project at I-84/Route 37, which will be adv

tised in June 2009. The cost of the bottleneck improvements is estimated to be $900,000.

I-84 Eastbound at Exit 7 (Route 7 Northbound) – Existing Lane Configuration

I-84 Eastbound at Exit 7 (Route 7 Northbound) – Proposed Lane Configuration
Major Reconstruction Project in New Hampshire

The existing 19.8-mile four-lane section of I-93 from the Massachusetts state line north to Manchester w

inally built in the 1960s and since that time, the facility has not undergone any major system wide upgrade

modernization. The interchanges at Exits 2, 3, and 5 remain essentially as they were originally constructe

Exits 1 and 4 were partially reconstructed in the late 80s and early 90s, but are rapidly becoming incapable 

meeting traffic demand. To address these problems, NHDOT identified a major, multiyear reconstructio

ect. The project will include: widening the mainline from two to four lanes per direction; widening of 44

bridges; reconstruction of all five interchanges on the section; addition of sound barriers; construction o

new park and ride lots; expanded bus service; and the purchase of right-of-way for a future light rail line. 

ever, because this project has an ambitious scale, it will take several years to complete and major congestio

occurs at Exit 5.

The State of New Hampshire has selected work at Exit 5 (Londonderry) on I-93 to be expedited in 

to address the bottlenecking concerns which currently exist. The signalized northbound and southb

ramps back up onto the mainline during peaks. The original project called for much of Exit 5 to be rec

structed in FY 2009. In partial response to the LBR program attention, improvements to two of the exis

ramps are being expedited. The southbound off ramp at Exit 5 will be extended. The shoulder area on t

mainline will be widened and reinforced to hold some of the queue. The signal at the top of the ramp w

coordinated with loop detection. Reconstruction or widening of two nearby bridges will be accomplished

estimated cost of this expedited work is $9.13 million. The completed project will meld into the overall 19.8

I-93 project

Continuous Flow Design Improves Intersection Operations in Utah 

A study of SR-171 (3500 South) at SR-154 (Bangerter Highway) in West Valley City showed that the inter

is one of the busiest in the State, with over 100,000 vehicles passing through the intersection on a typical 

day. Heavy rush hour traffic forces drivers to wait for up to four cycles of the traffic signal to get through 

intersection, which contributes to congestion and safety problems that ripple out to affect other intersecti

both 3500 South and Bangerter Highway. The study estimates that traffic through the intersection will in

31 percent by 2030.

Several improvement alternatives for the intersection were evaluated, including a grade separated intercha

However, UDOT decided to implement Utah’s first continuous flow intersection (CFI). An unconven

intersection design, CFIs reduce the steps in the traffic signal cycle and place left turning vehicles along 

safer path. Compared to a grade separated interchange, the CFI is a low cost alternative.

The CFI improvement project was started in March 2007 and was completed six months later in Septemb

2007, with a total project cost of $7.5 million.
42

The City of Los Angeles recently initiated this four-year program to identify and resolve bottlenecks at sign

ized intersections. The City is severely constrained in expanding mainlines and adding additional lanes at in

sections, as right-of-way costs are prohibitive. The program has identified 300 bottleneck locations to date.

Bottlenecks are defined as signalized intersections reaching greater than 40 percent loop occupancy on th

upstream system detectors for four 15-minute periods. Once a signal is identified as a bottleneck, the sig

timing and phasing is reviewed to see if these can be adjusted to meet demand better. If this is not fruit

then additional improvement types such as re-striping to provide additional lanes, parking restrictions, or

physical widening are evaluated:

The program initially identified 98 intersections that met the occupancy thresholds during peak periods: 

• Thirty six percent were successfully treated through signal retiming;

• Fourteen percent were identified as candidates for parking restrictions or lane additions;

• Eight percent were caused by ramp metering overflow from freeways, which will require coordination 

Caltrans to adjust metering rates and/or provide extra queue storage; and

• Forty two percent could not be reasonably treated by any of the strategies identified above; demand w

simply too high and improvements would be too costly.

Unfortunately, adaptive signal timing will not resolve the issues for the remaining bottlenecks on the arterial 

tem, as the competing approaches and saturation requires alternatives beyond adaptive timing. However, tim

is always the first approach to resolution of the bottlenecks identified through the program.

The City anticipates adding 1,000 more signals over the next 15 years with 300 to 400 in the more immed

future. The OBR program will be used in the long-range planning process to help identify funding sources 

solutions. Additionally, it will help building the case for changes both politically and within the community

about how bottlenecks are addressed and resolved.
Bottleneck Improvements

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 indefinite 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 benefit of demonstrating to the public that the agency is actively engaged in fighting

congestion.

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

example, the Minnesota DOT (MnDOT) 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 identification 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. For example, the Ohio DOT

(ODOT) added a congestion-based index ranking to their predominantly

safety-based Highway Safety Program (HSP). The result is that congestion

weighted problems now have a specific “voice” in the rankings of all candidate ODOT projects. The HSP was used as the mechanism because many of

its projects also are low-cost and quickly implementable.
• The California Department of Transportation (Caltrans) does no

formal bottleneck planning process; rather, bottleneck issues are add

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

operational planning is guided by the Transportation Management S

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

in improving congestion. In much of California’s metropolitan area

congestion is a 24/7 occurrence, and traffic management is a 24/7 j

tlenecks are tweaked “in real time” as part of their Corridor System 

ment Plans (CSMP), which are developed for some of California’s m

congested transportation corridors. System monitoring and evaluati

as the foundation for the entire process because it can not only ident

gestion problems, but also be used to evaluate and prioritize competi

ments. Caltrans does not have a direct funding for bottlenecks, altho

bottleneck projects are routinely programmed through the CSMP pr

• In Ohio, bottlenecks are part and parcel of the overarching Ohio D

ment of Transportation (ODOT) Highway Safety Program (HSP), 

ranks all candidate projects and drives the statewide highway projec

tion and scheduling process. Beginning in 2002, ODOT developed 

gestion mapping” division that uses V/C ratios developed from tra

recorders and roadway inventory. About the same time, ODOT ad

tion pushed for an annual process of overlaying congestion-index a

safety-index “hot spots.” As a result, congestion hot spots now hav

“voice” in the process regardless of crash indices, and congestion r

problems now compete for attention in the HSP listing. Specificall

way sections with V/C ratios greater than 1.0 are considered “cong

and are added to the listing. Sections with V/C between 0.9 and 1.

outside the cities of Columbus, Cincinnati, and Cleveland, are also 

After ODOT headquarters completes their statewide effort of con

mapping and safety indexing, the respective District engineers are re

for developing countermeasures for their top-listed candidate projec

trict Safety Review Teams sort projects into three scales – low (less t

$100K and quickly implementable), medium ($100K to $5M and 1 t

years), and high (greater than $5M and necessitating more than year

implement) – and then compete with other projects having the same 

in other districts.

• Minnesota DOT was originally driven to explore low-cost congesti

projects because of budgetary restrictions, but soon realized that th

ects could be implemented very quickly and, as a bonus, were highly 

44
which led to many effective projects in the following two years. MnDOT also

found that because of lower costs, it could identify multiple locations

throughout the region and “spread around” bottleneck reduction projects in a

fair and equitable manner. This process consisted of completing a study,

which included a five-step process to narrow potential projects into a recommendation list to the state legislature. Evaluation of completed projects produced high benefit/cost ratios, usually greater than 8:1.

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

program of about $5 million per year for the identification and implementation of low cost traffic congestion improvements at intersections. The

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

when a mega-project’s ‘no build’ alternative is chosen?” The program has

been well-received by the public and local governments. Projects typically

include low-cost projects that can be implemented quickly, such as signal

timing upgrades and adding turn lanes and through lanes at intersections.

The Maryland SHA has also has had considerable success with projects to

improve freeway ramps and merge 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 Office, begins with needs identification 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. Traffic data and the statewide model are used to identify deficiencies, but it is

the responsibility of the districts to identify and resolve hot spots.

• Washington State DOT (WSDOT) 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 benefit/cost ratio and other qualitative factors.
46

Step 1: Project Identification

Potential congestion management projects were identified from existing sources:

• Low-cost capacity improvements (e.g., auxiliary lanes)

• Re-striping lane configuration

• Traffic control device improvements (e.g., ramp meters and signal timing)

Step 2: Quantitative Screening

• Project cost < $15 million

• Not in three-year TIP

• Annual hours of delay > 25,000

• Minimum of two hours of congestion

Step 3: Qualitative Screening

• Design readiness

• Cost range

• Congestion benefit

• Construction traffic management

• Future demand changes

• No adverse downstream effects

Step 4: Expert Workshop

Projects were prioritized by an expert group during a half-day workshop.

Step 5: Project Planning

The following were prepared for each project:

• Geometric sketches

• Project scope

• Congestion impacts

• Safety impacts

• Benefit-to-cost ratio
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.
Auxiliary lanes – Typically, any lane whose primary function is not simp

through traffic. This can range from turn lanes, ramps, and other single p

lanes, or it can be broadened to imply that a traffic bearing shoulder can b

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

peak is over.

Bottleneck – There can be many definitions. Here are a few that are typ

used. 1) A critical point of traffic congestion evidenced by queues upstre

free flowing traffic downstream; 2) A location on a highway where there i

physical capacity, surges in demand (traffic volumes), or both; 3) A point 

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

usage of a highway section periodically exceeds the section’s physical ability 

it, and is independent of traffic-disrupting events that can occur on the roa

Capacity – The maximum amount of traffic capable of being handled by 

highway section. Traffic engineers usually speak in terms of “free flow” c

Congestion (specifically, traffic congestion) – FHWA’s Traffic Conge

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

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

much slower – than normal or free flow speeds. (Congestion is) stop-an

traffic. Previous work has shown that congestion is the result of seven 

causes,2 often interacting with one another.” Since a bottleneck is a cau

congestion, congestion cannot be solely analogous to a bottleneck. Con

is more. For example, a “congested” corridor may harbor multiple bott

or any combination of the seven root causes of congestion.

Downstream traffic – Traffic that is beyond (past) the subject point on a 

Hidden bottleneck – A highway location where some type of physical 

tion is present, but traffic flow into this area is metered by an upstream 

neck so the location does not appear as a bottleneck under prevailing co

Removal of the upstream bottleneck will cause the hidden one to emerg

new bottleneck.

48

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

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

superimposed on a system with a fixed 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 fir

another way, absent over-demand, there would just be “volume,” but not necessarily “congeste
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 traffic 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 traffic, a recurring event is a traffic 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 peak-hour 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.

Traffic microsimulation tools – Complex microsimulation tools that rely on input

of traffic 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 traffic demand.

Upstream traffic – Traffic that has not yet arrived at the subject point on a highway.
50

and delays due to recurring operational conditions or nonrecurring traffic-influencing 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 cas

work zones or special events, these occurrences are irregular and are not predictably habitual or recurring in lo

Causes: Recurring: Operational Causes – A “facility determinate” condition wherein a fixed condition (the design 

of the facility at that point) allows surging traffic confluence to periodically overwhelm the roadway’s physical 

capacity) to handle the traffic, resulting in predictable periods of delay.

Nonrecurring: Dynamic Occurrences – An “event determinate” occurrence, wherein a dynamic situatio

reduces available capacity (e.g., loss of lanes due to incident or work zone) or increases demand (e.g., specia

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

lane reduction, curves, poorly operating traffic signals.

Nonrecurring: Work zones, crashes, incidents, special events, weather.

Supplementary Terms: 

(applies to either type)

“Active” bottlenecks – When traffic “released” past the bottleneck is not affected by a downstream restrictio

queue spillback) from another bottleneck. “Hidden” bottlenecks – When traffic demand is metered by anoth

bottleneck(s); i.e., either a lesser or nonexistent bottleneck that would increase or appear, respectively, if only u

traffic could reach it.

Identification of: 

(applies to either type)

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

engineers and agencies also measure performance data: average speed (travel time), lane densities, queue leng

discharge rates, vehicle miles of travel (VMT), and vehicle hours of travel (VHT). 

Measurement of: 

(applies to either type)

Data is collected using manual techniques (e.g., floating cars, aerial photography, or manual counts from video re

or from dynamic surveillance (e.g., detectors, radar, video, etc.) collected in real time. Modeling, especially micro

can be used to study the impacts of bottleneck remediation on upstream and downstream conditions.

Classification of: Recurring: Type I – Demand surge, no capacity reduction (typically at freeway on-ramp merges). Type II – Ca

reduction, no demand surge (typically changes in freeway geometry; lane drop, grade, curve). Type III – Combin

surge and capacity reduction (typically in weaving sections).

Nonrecurring: Usually classified by the type of event (e.g., incident, work zone) and severity of impact (e.g., d

the number of lanes lost, closed, or impassable).

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

rear of the queue as volume declines.

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

The bottleneck clears from the front or rear of the queue, depending on whether the cause is incident-related 

or volume-related (latter), respectively.

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

ditions return).

Nonrecurring: When dynamic event is removed; queue should dissipate, thereafter.
and delays due to recurring operational conditions or nonrecurring traffic-influencing events.

Practical 

Mitigations: Recurring: Corridor Congestion Recurring: Localized Bottlenecks Nonrecurring

Dynamic pricing Use shoulder lane Improve incident response capabilities; 

dent impact; reduce on scene time for c

dents; reduce facility “downtime” durin

In work zones, maintain maximum num

lanes during peak times; shorten duratio

innovative methods and contracting pra

mize number of times a section is an ac

zone by combining improvements (e.g.

safety) and using highly durable materia

least intrusive detour(s).

Pre-plan for and coordinate special eve

quately and efficiently handle event traf

not only the main event but the subord

ies, VIP access, emergency response, an

post-event activities.

Have predetermined detour plans for p

tions of highway in the event of weath

dent-related events, including available 

arrows, sign stands, VMS boards, publi

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

Traffic 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)
52

NOTICE

This document is disseminated under the sponsorship of the U.S. Department of

Transportation in the interest of information exchange. The U.S. Government

assumes no liability for the use of the information contained in this document.

This report does not constitute a standard, specification, or regulation.

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

or manufacturers’ names may appear in this report only because they are considered essential to the objective of the document.

The publication of this document was sponsored by the U.S. Federal Highway

Administration under contract DTFH61-06-00004, awarded to Cambridge

Systematics, Inc.

QUALITY ASSURANCE STATEMENT

The Federal Highway Administration (FHWA) provides high-quality information

to serve Government, industry, and the public in a manner that promotes public

understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews

quality issues and adjusts its programs and processes to ensure continuous quality

improvement.
June 2009