U.S. Department of TransportationFederal Highway Administration
23

Capacity – The maximum amount of traffic capable of being handled by a given highwaysection. Traffic engineers usually speak in terms of “free flow” capacity.Congestion – The FHWA “Traffic Congestion Reliability” reports define congestion as an

excess of vehicles on a portion of roadway at a particular time resulting in speeds that are slower – sometimesmuch slower – than normal or free flow speeds. (Congestion is) stop-and-go traffic. Previous work has shownthat congestion is the result of seven root causes2 often interacting with one another. Since a bottleneck is a

cause of congestion, congestion cannot be solely analogous to a bottleneck. Congestion ismore. For example, a “congested” corridor may harbor multiple bottlenecks or any combination of the seven root causes.Downstream traffic – Traffic that is beyond (past) the subject point on a highway.Hidden bottleneck – A highway location where some type of physical restriction is present,but traffic flow into this area is metered by an upstream bottleneck so the location does notappear as a bottleneck under prevailing conditions. Removal of the upstream bottleneck willcause the hidden one to emerge as a new bottleneck.

2 The seven root causes are bottlenecks (a.k.a. “capacity constraints”), incidents, work zones, weather, poorly timed signals 

et.al., special events, and over-capacity demand (i.e., daily and seasonal peaks superimposed on a system with a fixed capacity).Some sources cite only six root causes because they see over-demand as an inherent subelement necessary for any of theother causes to exist in the first place. Put another way, absent over-demand there would just be “volume,” but not necessarily“congested” volume.
1

The delays arising from traffic congestion are a fact of life in many communities.According to a February 2007 Harris Poll, just over one-third (37 percent) cite traffic congestion as a serious problem in the community, while one-quarter say trafficcongestion is a serious problem that is not being addressed. Close to half of allcongestion happens day after day at the same time and location. Much of thisrecurring congestion is due to physical bottlenecks – specific points on the highwaysystem where traffic flow is restricted.While many of the Nation’s bottlenecks can best be addressed through costly majorconstruction projects, there is also significant opportunity for the application ofoperational and low-cost infrastructure solutions to bring about relief in the shortterm. This document, Traffic Bottlenecks: A Primer – Focus on Low-Cost Operational

Improvements, describes bottlenecks 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. While updated hardcopy versions will occasionally be made available, an electronic version will be regularly supplemented as new strategies are identified from within the transportationcommunity. The Primer is a key resource for Federal Highway Administration’sLocalized Bottleneck Reduction (LBR) Program, providing a virtual forum for peerexchange between members of the transportation community interested in alleviating bottleneck congestion. The LBR program, initiated in 2007, is designed toexpand the portfolio of bottleneck reduction tools available to transportation agencies to encompass innovative, readily adopted strategies for reducing congestion atbottleneck locations.

ne, or other atypical event.

way via use of control devices

of vehicles entering or leaving

nt of ramp metering is to

ough vehicles. Done proper

ons.

a traffic condition (i.e., a bot

on and at the same time daily,

peak-hour slowdowns at

ese events because one

he same manner and place.

ols that rely on input of traf

parameters of each input, in

ngineers can test different

t point on a highway.
mension – duration,

by traffic congestion has

etch from 2 or 3 hours in

on people. Larger areas

cent of travel. This sta

Many cities have a few

raffic. Weekend traffic

r major shopping centers

delay penalty (the extra

om 13 to 37 percent in the

37 percent more travel

percent 10 years earlier.

g trips, doctor visits, and

both monetary and socie

 Texas Transportation

ed to be $63 billion based

f wasted fuel calculated in

 2003 was equivalent to

reover, these congestion

e the growth rate of the

d to rise to 1.6 percent of

ss than half of the overall

lude:

ies outside of the 85-city

es and labor market sizes;

23

Active bottleneck – When traffic flow through the bottleneck is not (further) affected bydownstream restrictions.Auxiliary lanes – Typically, any lane whose primary function is not simply to carry throughtraffic. This can range from turn lanes, ramps, and other single-purpose lanes, or it can bebroadened to imply that a traffic-bearing shoulder can be opened in peak-periods to help alleviate a bottleneck, and then “shut back off” when the peak is over.Bottleneck – There can be many definitions. Here are a few that are typically used.1) A critical point of traffic congestion evidenced by queues upstream and freeflowing traffic downstream; 2) A location on a highway where there is loss ofphysical capacity, surges in demand (traffic volumes), or both; 3) A point wheretraffic demand exceeds the normal capacity; and 4) A location where demand forusage of a highway section periodically exceeds the section’s physical ability tohandle it, and is independent of traffic-disrupting events that can occur on theroadway.Capacity – The maximum amount of traffic capable of being handled by a given highwaysection. Traffic engineers usually speak in terms of “free flow” capacity.Congestion – The FHWA “Traffic Congestion Reliability” reports define congestion as anexcess of vehicles on a portion of roadway at a particular time resulting in speeds that are slower – sometimesmuch slower – than normal or free flow speeds. (Congestion is) stop-and-go traffic. Previous work has shownthat congestion is the result of seven root causes2 often interacting with one another. Since a bottleneck is a

cause of congestion, congestion cannot be solely analogous to a bottleneck. Congestion ismore. For example, a “congested” corridor may harbor multiple bottlenecks or any combination of the seven root causes.Downstream traffic – Traffic that is beyond (past) the subject point on a highway.Hidden bottleneck – A highway location where some type of physical restriction is present,but traffic flow into this area is metered by an upstream bottleneck so the location does notappear as a bottleneck under prevailing conditions. Removal of the upstream bottleneck willcause the hidden one to emerge as a new bottleneck.

2 The seven root causes are bottlenecks (a.k.a. “capacity constraints”), incidents, work zones, weather, poorly timed signals 

et.al., special events, and over-capacity demand (i.e., daily and seasonal peaks superimposed on a system with a fixed capacity).Some sources cite only six root causes because they see over-demand as an inherent subelement necessary for any of theother causes to exist in the first place. Put another way, absent over-demand there would just be “volume,” but not necessarily“congested” volume.
3

• Costs to passengers of having to leave early for a destination because of

congestion-related unreliability in travel times.

Not only does congestion dampen the economy, it also impacts the way we live:Parents miss events with their children; friends and families find it harder to spendtime together; and civic participation is increasingly difficult. Evidence suggeststhat each additional 10 minutes in commuting time cuts involvement in community affairs by 10 percent.The root causes of congestion have long been understood, and there is nowbroad consensus that congestion generally reflects a fundamental imbalance ofsupply and demand. That is, during hours of peak usage of the transportationfacilities most desirable to motorists, the supply of, for example, roadway capacityis insufficient to meet the demand for those facilities. Economists have longunderstood that such an imbalance stems from inefficient pricing, where thetrue costs of usage are not reflected in prices paid by the users. For example,travelers are not generally charged for the impact their trip will have on othersusing the same facility (e.g., increased levels of congestion) or on other membersof society (e.g., increased air pollution). In fact, in this country, access to highwaytravel is, for the most part, rationed by delay.The imbalance of supply and demand leading to congestion is also impacted bythe absolute volume of traffic (e.g., demand) on a given facility relative to its physical capacity (e.g., supply). When we look at traffic congestion from a demandperspective, we are looking at how many vehicles compete for space on a particular facility at a given time. The demand for a facility is a function of individualdecisions as to when, where, how, and even if highway travel will take place.

ds, we will take the oppor

t one of several program

re information may be

Washington Post, 

June 2007

A June 2007 article in theWashington Post laments the difficulty parents in outlying suburbshave in attending their kids’evening soccer, t-ball, baseball,and softball games. Parents whoare coaches, and have responsibility to be early to set up the field,have an especially hard time,often arriving in their officeclothes directly from work.Games are routinely pushed back30 to 60 minutes; leagues areover taxed because only one, andnot two, games can be played perevening on the same field. 

n the past few years. The

he I-75/I-85 merge just

ntified as the nation’s sixth

ottleneck study.

o add ramp storage and

d and Ellis Street south

ramp meters in that sec

d Avenue). The ramp

ent in time for the four

ngestion hours was

Locally Known as the

-Period

om Memorial Drive.
21

this capacity. Congestion

ither “recurring” or 

community of trans

mitigate or reduce conges

 time and place on the

 is simply not adequate 

other hand, nonrecurring

c incidents, and bad

cur on an already con

nts a pie chart showing

Maryland’s Quick Fix at Interstate-70/Intestate-695A recent project at the I-70/I-695 (Baltimore Beltway) interchange outside of Baltimore was discussed. The eastbound approachfrom I-70 to I-695 backed up on to the mainline of I-70, restricting through traffic. Widening the entire ramp would have beenvery expensive due to the need for major bridge reconstruction. Instead the ramp was widened up to the bridge. This providedadequate storage to relieve the backup on to the mainline and did so at a reasonable cost.

Inadequate

Physical

Capacity

(Bottlenecks)

40%

Traffic Incidents 

25%

htm.

The interchange of I-75 and Bruce B. Downs Boulevard in suburban Tampa exhibited the characteristics of a bottleneckamenable to a low-cost improvement. One of the major traffic flows is a right turn from the ramp and a quick left turn onto anarterial. The weaving on the cross street caused queuing on the ramp which often backed up to the freeway mainline. The problem was addressed by adding a free right turn lane and a signalized right turn lane. The traffic that needed to make the quick leftturn is signed to use the signalized right turn lane. Queuing on the mainline is no longer a problem.

TxDOT District Freeway(s) And Limits Description of Bottleneck Improvement(s)

Dallas EB IH 30, IH 35E to IH 45 Changed exit ramp to Harwood to become entrance ramp from the 

collector-distributor road.

Fort Worth NB SH 360 @ Division (SH180) Converted outside shoulder to auxiliary lane between two closely spaced

exit ramps.

Dallas NB IH 35E, IH 30 to Dallas

North Tollway

Addition of two auxiliary lanes by inside shoulder conversion.

El Paso EB IH 10 @ U.S. 54 Restriped one-lane ramp to two lanes, dropped main lane at exit, added at

entrance, added auxiliary lane.

Dallas EB SH 190 to SB U.S. 750 Restriped to give entrance ramp from SH 190 its own lane onto U.S. 75.Dallas NB IH 35E Ramp to Dallas North

Tollway 

Restriped merge to allow ramp its own lane onto approach to tollbooths.

Dallas NB-SB IH 35E, LP 12 to IH 635 Converted inside shoulders to travel lanes for 3.0 miles and removed two

inside merges.

Dallas WB IH 30 Ramp to SB IH 35E Restriped to balance freeway capacity with freeway volumes at merge.Fort Worth EB IH 20 to NB SH 360 Added deceleration lane to IH 20 before exit; dropped main lane at exit,

added back at SH 360 entrance ramp.

Fort Worth SB SH 360 to WB IH 20 Auxiliary lanes on SH 360, dropped main lane on IH 20 at SH 360 exit,

added lane at SH 360 entrance.

Fort Worth SB SH 360 @ Division Closed entrance ramp, forcing traffic through signal, added 

auxiliary lane to next entrance.

Dallas EB IH 635 to NB U.S. 75 Widened and restriped left-side ramp from one to two lanes.Dallas SB U.S. 75 to WB IH 635 Converted inside shoulder on IH 635 to allow 

ramp from U.S. 75 its own lane.

Low-Cost Bottleneck Improvements in Texas. The Texas Department of Transportation (TxDOT) has undertaken significant low-cost freeway bottleneck improvements in recent years. These include:

Source: Walters, Carol H, Cooner, Scott A., and Ranft, Stephen E., Looking Again at Bottlenecks on Freeways Evaluating Case Studies in Texas, 

November 15, 2004, http://pubsindex.trb.org/document/view/default.asp?lbid=775977.
5

Consistent with the paradigm of its causes, there are four distinct, but nonethelessrelated strategies available to attack congestion. Table 1 outlines the four strategies and provides examples of specific options for furthering each of them. It isimportant to note that the strategies, while individually of merit, work best whenimplemented as a coordinated package of tools.Bring Supply and Demand into Alignment through Congestion Pricing.Congestion pricing or peak-period pricing, entails fees or tolls for road use thatvary by level of vehicle demand on the facility. As with market pricing in othersectors, road pricing helps allocate limited supply – in this case that of availableroad space.

Please Dispose of 

Gasoline Properly!

Imagine purchasing from 8 gallonsof fuel (in a small urban area) to36 gallons of fuel (in a largeurban area) per year, and thenthrowing it away. In fact, that isone estimate of how much fuel iswasted while idling in congestionper year. At $3.00/gallon, thatamounts to $24 to $108 “out ofpocket” for every driver in theseareas. The average was 28 gallons, or $84. 

2005 Urban Mobility Study, The Texas

Transportation Institute (TTI), 2005

Bring Supply and Demand into Alignment through Congestion PricingRoad Pricing Ramp MeteringCorridor ManagementProvide Real-Time Travel InformationProvide Better Choices as to How,When, Where, and If to TravelProvide More Attractive Alternatives to Single Occupant Vehicle Transportation (Including Better Transit, More Telecommuting, or High Occupancy Toll Lanes)Provide Real-Time Travel InformationStrategically Invest in New Transportation CapacityAdd New Construction on New AlignmentImprove the Management and Operation of the SystemQuickly Restore Capacity After Traffic Disrupting Events

– Improve the Management of Traffic Incidents– Improve Mobility at Work Zones– Respond Effectively to Inclement Weather Conditions– Plan Ahead for Special Events

Improve the Day-to-Day Operation of the System

– Improved Traffic Signal Timing– Operational and Low-Cost Construction Improvements to Relieve Bottlenecks (e.g., restriping)

Provide Real-Time Travel Information to Agencies and System Users

Table 1. Examples of Strategies to Reduce Highway Congestion

ost treatments can provide

of improvements have

e emerging. This section

sing these strategies to

ction programs;

ton State;

a;

k;

Atlanta Downtown 

ms in the Puget Sound

d system for two

 meters allow the freeway

vehicles has resulted in

t ramp from I-405 to south

d the traffic volumes han

he addition of a “weaving

stinations. Where traffic pat

o be relieved and provide a

national port business.

peeds. An additional lane
19

ly assessed electronically

n facilities. Road-use

vide incentives to shift

alternative modes; or 

ther trips or simply to 

rst, it seeks to balance

space. Second, it seeks

aintaining, and expanding

nd. Third, it seeks to

bjective may include rev

nd If to Travel. The goal

 a given road. This may

s such as employee

e attractive to use. Also

grams that, for example,

y. Although there is sig

city, concerns about air

ay of expanding the sys

normously expensive and

ew construction that

er to preserve or improve

or implemented by the interviewed agencies were:

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

In general, the interview responses to this question suggest there is a trend towardsfavoring the implementation of ramp metering as a low-cost bottleneck improvement. Such systems are particularly attractive since they allow traffic managers to directly control freeway demand levels. This unique capability providesa valuable tool for managing freeway bottlenecks that does not involve large andexpensive capacity expansions.Some of the key questions and considerations in the development of improvementalternatives for bottleneck removal include:

• Is there an inside shoulder that would create a usable traffic lane for a short

section of freeway?

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

while allowing adequate clearance to barriers (2 feet) and an outside shoulder? If not, are they short enough that a loss of shoulder as a breakdownlane 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 mustbe resolved?

• Are vertical clearance issues, grade-matching, and sight distance problems created?• If a shoulder is considered for conversion, is there right-of-way (ROW) to

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

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

traffic which is affected by it be better accommodated?

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

breakdown again or is this truly the clearing up of a “kink” in the system,without being a capacity addition which will overload some other part ofthe facility?1

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

Freeway Bottleneck Analysis Methodology.
7

may be addressed through increased expenditures. Environmental concerns may bemitigated and physical challenges overcome (for example, through tunneling).However, the resources to fund such improvements simply are not availablethrough traditional sources. For this reason, many professionals in the transportation community are enthusiastic about the opportunities potentially afforded bypublic-private partnerships and road pricing.Improve the Management and Operation of the System. This area of interest involves better managing the vehicles that are actually on the road, and the roaditself. “Smart” roads, traveler information, and improvements to the managementand operation of the facility are options available for using the available systemmore productively and bringing it to peak performance. Management and operations strategies are targeted at managing temporary disruptions (e.g., incidents) in away that will return the system to full capacity quicker; ensuring more efficient dayto-day operations through coordinated and up-to-date traffic signal timing and operational improvements to relieve bottlenecks; and providing real-time informationabout the system so that travelers can make immediate decisions about when, where,and how to travel, and transportation agencies can make real-time adjustments toimprove system operations.Effective and efficient management and operation of the system is foundationalto all of the above congestion reduction strategies. This is true because as trafficvolumes have grown over time and physical capacity has remained relatively constant, the system has become less able to absorb “surprise” – or nonrecurringevents. In the realm of managing the highway system, the margin for error is verysmall and continues to decline. In addition, operational fixes to the system arealso helpful in addressing the recurring congestion resulting from bottlenecks andimproper traffic signal timing. This is particularly significant in view of the factthat bottlenecks account for 40 percent of congestion and are often difficult toresolve via major reconstruction.

How Are Freight Delays 

Costly to You?

• Congestion means longer travel

times and increased costs inwasted fuel and driver remuneration. To compensate, companies add vehicles, hire moredrivers and employees, andextend their hours to accommodate us, eventually passingthrough these costs to shippersand consumers.

• FHWA estimates these

increases (to the company)to be between $25 and $200per hour depending on theproduct carried.

tion thresholds are

impending. This mostly

ave” that results through

 to bottlenecks and choke

.

r and smooth merges. A

 be 20th to merge into the

jumpers that bull ahead,

nes until they force their

quires on-site enforce

now the process and are

ement principles.

es.

mand into alignment. As

ons are relaxed, pricing

actitioners as a powerful

 address bottleneck 

Cooperative Highway

nts for Recurring Freeway

transportation personnel

me the low-cost improve

ns in their jurisdictions.

s showed that agencies

ks, most of them low

ategies include making

ctive additions to it.
17

U.S. DOT) announced the

ransportation Network

itiative is focused on

estion. Because of this

ucing congestion, not

Each area includes activi

short term and to build

duction efforts.

er into Urban Partnership

, bold, and innovative

 important to note that

mprehensive package that

 alternatives, such asnsportation; ensuring that

oper technology is in

he pricing strategy.

Department is working

nt in the construction,

re.

ents. The Department is

cal improvements aimed

ng state efforts to 1) pro

incident management

ms and quick clearance

mprove work zone safety

w-cost construction

Short-Term, Low-Cost Improvements Here is a sampling of operational remediations.

1. Using a short section of shoulder as an additional travel lane.2. Re-striping merge or diverge areas to better serve demand.3. Reducing lane widths to add a travel and/or auxiliary lane 

(e.g., re-striping).

4. Modifying weaving (e.g., adding collector/distributor or through lanes).5. Metering or closing entrance ramps.

Bottleneck

Types

Mitigation Measures

Auxiliary

Lanes

CollectorDistributor

Road

Paved Right

Shoulder

Paved Left

Shoulder

Shoulder/

Plus Lane

Re-Stripping

to Add More

Narrow Lanes

All Purpose

Lane

(Concurrent

or Reversible)

HOV Lanes

(Concurrent

or Reversible)

Truck

Reversible

Ramp

Metering

Temporary

Ramp

Closures

Traffic

Diversion

Information

Heavy OnRamp Demand ++ - + - ++ + ++ - + ++ - +WeavingSections + + + + + ++ ++ ++ + - - +Lane Drops ++ - ++ - ++ ++ ++ ++ + + + +Tunnels andBridges - - - - - ++ - + ++ - - +Horizontal andVertical Curves ++ - ++ ++ ++ - ++ + ++ + + +Narrow Lanesand LateralObstruction

+ + + + + - + + ++ + + +

InadequateAcceleratedand/orDeceleratedLanes

++ ++ ++ - ++ + + + ++ + ++ ++

Table 3. Mapping Bottleneck Problems to Mitigation Measures

++ = good solution + = may be helpful - = not applicable

Source: Adapted from interim materials from NCHRP Project 3-83.
9

accelerating the development of multistate, multiuse transportation corridors byrunning a competition to select three to five major growth corridors in need oflong-term investment.Target Major Freight Bottlenecks and Expand Freight Policy Outreach.The Department is working to find and implement solutions to freight transportation and border congestion that will facilitate trade and travel without compromising either highway safety or the vital mission of securing America’s borders. This area of interest emphasizes a Southern California Freight Outreacheffort that will broker consensus on immediate and longer-term transportationsolutions by bringing together key stakeholders.Accelerate Major Aviation Capacity Projects and Provide a Future FundingFramework. The Department is working to address congestion in the aviationsystem by designing and deploying the Next Generation Air TransportationSystem. In addition, the Department will advance reforms that lead to bettermanagement of airport and airspace congestion.

ttleneck locations and the

 congestion is prevalent,

g, and analyzing bottle

 can mathematically iden

ngestion or poor level of

a to identify where and

Historical data is used to

mplished with a range 

pected to be bottlenecks

ls can provide a detailed

can assist in determining

ottleneck analysis, care

n doesn’t just transfer the

may be “metering” flow

s acceptably, but the

ttleneck.

mprovements are inclusive

ongly considered.

es, the queue formed by a

tream of it. Improving

n locations. It is impor

dden bottlenecks” exist at

s are included. There are

f geometrics and abrupt

yzed by analysis tools.

ed to identify those prob

oblems and potential solu

k improvements can be

funding is secured.
15

oadways that routinely

 volumes exceed highway

dated by base capacity

characterized by queues

n carry only so much

behind it, much the same

on is even worse for traf

o conditions, capacity is

tleneck because of the

“Bottlenecks are caused only by not enough lanes on an extended highwaysection.”In the past, recurring congestion was felt to be exclusively a systemic problem (e.g., notenough lanes, a system widening is the only solution), but often, clearing unique bottlenecklocations within the system demonstrates that the uniform highway segments may not necessarily be underdesigned.Traditional capital solutions often grew from the misconception that a multilane facilityshould be designed to alleviate the recurring peak hours each day. The problem is thatfunding for these large scale projects is limited, and right-of-way is often restricted, suchthat these projects take a long time (many years) to complete. As a result, recurring congestion historically went untreated, or at least competed against other worthy projects,until funding became available to “catch up” to the problem that had grown from theday the facility opened.With a shift in the focus away from perceiving that recurring congestion is systemic (andthus treatable with only large projects), we must explore a wider-range of improvementstrategies that are possible in the short-term. While these will never replace the need forcorridor-wide fixes – especially at the “mega-bottlenecks” such as major freeway-to-freeway interchanges – bottleneck-specific improvements can provide congestion relief.“Bottlenecks can’t be fixed without massive reconstruction.”With the focus of transportation planners on major capital projects, it has been assumedfor many years that bottlenecks cannot be fixed without massive reconstruction of aninterchange or corridor. There are numerous examples where agencies opted to makelower-cost improvements that made significant improvement 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 specialevents can happen at any time, congestion is not restricted to peak times of the day. Theimprovements made at bottlenecks primarily to address peak-period problems will carryover to the times outside of the peak when congestion occurs.

Demand-Related

Systemic High Demand
11

ngestion?

relieve congestion at bot

h projects is usually enor

ely low-cost geometric

arrow, high-occupancy

p metering) can be imple

performance by introduc

 Maryland State Highway

 $5.0 million per year for

 congestion improve

ved by the public and

x” type projects that can

ing turn lanes and

also has had considerable

rove freeway merge areas

ther States have also been

Germany, Denmark, the

 their respective conges

reas, the majority of the

ices, some of the knowl

lenecks and chokepoints.

tegies to relieve bottleneck

ing recurring congestion

rupting events at bottle

ocks a single-lane of

e impact would be greater

nly affect recurring conges

of this is that strategies to

onrecurring events.

onrecurring events,

n travel time reliability.

Some bottlenecks were originally constructed many years ago using designs thatare now considered to be antiquated. Others that have been built to extremelyhigh design specifications are simply overwhelmed by traffic. Whatever the rootcause, operational conflicts can occur at:

• A “lane drop,” where one or more traffic lanes are lost. These typically

appear at bridge crossings and in work zones. The latter, however, is anonrecurring event and is usually remedied when the work zone isremoved.

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

access entry or exit ramps.

• “Freeway on-ramps” are merging areas where traffic from local streets

can join a freeway.

• “Freeway-to-freeway interchanges” are special cases of on ramps where

flow from one freeway is directed to another. These are typically themost severe form of physical bottlenecks because of the high traffic volumes involved.

• “Abrupt changes in highway alignment” occur at sharp curves and hills

and cause drivers to slow down either because of safety concerns orbecause their vehicles cannot maintain speed on upgrades. Anotherexample of this type of bottleneck is in work zones where lanes may beredirected or “shifted” during construction.

• “Intended interruptions to traffic flow” are literally “traffic disruptions

on purpose” that are sometimes necessary in order to manage systemflow. Traffic signals, freeway ramp meters, and tollbooths are all examples of this type of capacity loss.

Opportunities for Lower

Cost Solutions

Respondents to the FHWA DivisionOffice survey said that 71 percentof bottlenecks were on freeways.Further, 42 percent were interchange related and 36 percenthad no specific improvement orplan underway. Full interchangeor freeway reconstructions werethe most commonly perceivedsolutions. However, at least aportion of those might qualify forlow-cost response actions that canbe implemented in the short-termto improve traffic flow.Recent discussions with severalstate partners have reinforcedtheir position. Specifically, highranking state DOT officials andmid-level staffers engaged in dayto-day operations have all opinedthat there are “tremendous” and

“significant” benefits to pursuing

low-cost operational improvements. These benefits range fromthe direct (reductions in delay,increases in traffic throughput) tothe indirect (public confidence andagency image-boosting effects). 
13

ovements – and of travel

highway system, there are

on a daily basis. These

eway-to-freeway), a series

points for congestion in

on in corridors where 

s such as:

ve come to realize that

ational studies have high

ban areas. These studies

 problem, warranting that

oblem.

ed two studies of the

es produced rankings of

and discussed what was

ments had been scheduled.

s are major freeway-to

dy updated the rankings

ories” – bottlenecks iden

struction.

ght (trucking) arena.

of bottlenecks on truck

also were considered (e.g.,

terms of total delay, the

lated – also are the major

nce of bottlenecks as well.

dy of freight (trucking)

dertaking a study of all

onal Commission has

on Management Process

twork.

Rank City Freeway Location

Annual Hours of

Delay Hours 

(in Thousands)

1 Los Angeles U.S. 101 U.S.-101 (Ventura Freeway) at I-405 Interchange 27,1442 Houston I-610 I-610 at I-10 Interchange (West) 25,1813 Chicago I-90 I-90/94 at I-290 Interchange (“Circle Interchange”) 25,0684 Phoenix I-10 I-10 at SR-51/SR-202 Interchange (“Mini-Stack”) 22,8055 Los Angeles I-405 I-405 (San Diego Freeway) at I-10 Interchange 22,7926 Atlanta I-75 I-75 south of the I-85 Interchange 21,0457 Washington (D.C.-

Maryland-Virginia)

I-495 I-495 at I-270 Interchange 19,429

8 Los Angeles I-10 I-10 (Santa Monica Freeway) at I-5 Interchange 18,6069 Los Angeles I-405 I-405 (San Diego Freeway) at I-605 Interchange 18,606

10 Atlanta I-285 I-285 at I-85 Interchange (“Spaghetti Junction”) 17,072

11 Chicago I-94 I-94 (Dan Ryan Expressway) at I-90 Skyway Split (Southside) 16,713

12 Phoenix I-17 I-17 (Black Canyon Freeway) at I-10 Interchange (the “Stack”) to Cactus Road 16,31013 Los Angeles I-5 I-5 (Santa Ana Freeway) at SR-22/SR-57 Interchange (“Orange Crush”) 16,304

14 Providence I-95 I-95 at I-195 Interchange 15,340

15 Washington (D.C.

Maryland-Virginia)

I-495 I-495 at I-95 Interchange 15,035

Source: Unclogging America’s Arteries: Effective Relief for Highway Bottlenecks, American Highway Users Alliance, February 2004. Only those 

bottlenecks that cause an estimated 10 million hours of annual delay are included in this list.

y
13

ovements – and of travel

highway system, there are

on a daily basis. These

eway-to-freeway), a series

points for congestion in

on in corridors where 

s such as:

ve come to realize that

ational studies have high

ban areas. These studies

 problem, warranting that

oblem.

ed two studies of the

es produced rankings of

and discussed what was

ments had been scheduled.

s are major freeway-to

dy updated the rankings

ories” – bottlenecks iden

struction.

ght (trucking) arena.

of bottlenecks on truck

also were considered (e.g.,

terms of total delay, the

lated – also are the major

nce of bottlenecks as well.

dy of freight (trucking)

dertaking a study of all

onal Commission has

on Management Process

twork.

Rank City Freeway Location

Annual Hours of

Delay Hours 

(in Thousands)

1 Los Angeles U.S. 101 U.S.-101 (Ventura Freeway) at I-405 Interchange 27,1442 Houston I-610 I-610 at I-10 Interchange (West) 25,1813 Chicago I-90 I-90/94 at I-290 Interchange (“Circle Interchange”) 25,0684 Phoenix I-10 I-10 at SR-51/SR-202 Interchange (“Mini-Stack”) 22,8055 Los Angeles I-405 I-405 (San Diego Freeway) at I-10 Interchange 22,7926 Atlanta I-75 I-75 south of the I-85 Interchange 21,0457 Washington (D.C.-

Maryland-Virginia)

I-495 I-495 at I-270 Interchange 19,429

8 Los Angeles I-10 I-10 (Santa Monica Freeway) at I-5 Interchange 18,6069 Los Angeles I-405 I-405 (San Diego Freeway) at I-605 Interchange 18,606

10 Atlanta I-285 I-285 at I-85 Interchange (“Spaghetti Junction”) 17,072

11 Chicago I-94 I-94 (Dan Ryan Expressway) at I-90 Skyway Split (Southside) 16,713

12 Phoenix I-17 I-17 (Black Canyon Freeway) at I-10 Interchange (the “Stack”) to Cactus Road 16,31013 Los Angeles I-5 I-5 (Santa Ana Freeway) at SR-22/SR-57 Interchange (“Orange Crush”) 16,304

14 Providence I-95 I-95 at I-195 Interchange 15,340

15 Washington (D.C.

Maryland-Virginia)

I-495 I-495 at I-95 Interchange 15,035

Source: Unclogging America’s Arteries: Effective Relief for Highway Bottlenecks, American Highway Users Alliance, February 2004. Only those 

bottlenecks that cause an estimated 10 million hours of annual delay are included in this list.

y
11

ngestion?

relieve congestion at bot

h projects is usually enor

ely low-cost geometric

arrow, high-occupancy

p metering) can be imple

performance by introduc

 Maryland State Highway

 $5.0 million per year for

 congestion improve

ved by the public and

x” type projects that can

ing turn lanes and

also has had considerable

rove freeway merge areas

ther States have also been

Germany, Denmark, the

 their respective conges

reas, the majority of the

ices, some of the knowl

lenecks and chokepoints.

tegies to relieve bottleneck

ing recurring congestion

rupting events at bottle

ocks a single-lane of

e impact would be greater

nly affect recurring conges

of this is that strategies to

onrecurring events.

onrecurring events,

n travel time reliability.

Some bottlenecks were originally constructed many years ago using designs thatare now considered to be antiquated. Others that have been built to extremelyhigh design specifications are simply overwhelmed by traffic. Whatever the rootcause, operational conflicts can occur at:

• A “lane drop,” where one or more traffic lanes are lost. These typically

appear at bridge crossings and in work zones. The latter, however, is anonrecurring event and is usually remedied when the work zone isremoved.

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

access entry or exit ramps.

• “Freeway on-ramps” are merging areas where traffic from local streets

can join a freeway.

• “Freeway-to-freeway interchanges” are special cases of on ramps where

flow from one freeway is directed to another. These are typically themost severe form of physical bottlenecks because of the high traffic volumes involved.

• “Abrupt changes in highway alignment” occur at sharp curves and hills

and cause drivers to slow down either because of safety concerns orbecause their vehicles cannot maintain speed on upgrades. Anotherexample of this type of bottleneck is in work zones where lanes may beredirected or “shifted” during construction.

• “Intended interruptions to traffic flow” are literally “traffic disruptions

on purpose” that are sometimes necessary in order to manage systemflow. Traffic signals, freeway ramp meters, and tollbooths are all examples of this type of capacity loss.

Opportunities for Lower

Cost Solutions

Respondents to the FHWA DivisionOffice survey said that 71 percentof bottlenecks were on freeways.Further, 42 percent were interchange related and 36 percenthad no specific improvement orplan underway. Full interchangeor freeway reconstructions werethe most commonly perceivedsolutions. However, at least aportion of those might qualify forlow-cost response actions that canbe implemented in the short-termto improve traffic flow.Recent discussions with severalstate partners have reinforcedtheir position. Specifically, highranking state DOT officials andmid-level staffers engaged in dayto-day operations have all opinedthat there are “tremendous” and

“significant” benefits to pursuing

low-cost operational improvements. These benefits range fromthe direct (reductions in delay,increases in traffic throughput) tothe indirect (public confidence andagency image-boosting effects). 
15

oadways that routinely

 volumes exceed highway

dated by base capacity

characterized by queues

n carry only so much

behind it, much the same

on is even worse for traf

o conditions, capacity is

tleneck because of the

“Bottlenecks are caused only by not enough lanes on an extended highwaysection.”In the past, recurring congestion was felt to be exclusively a systemic problem (e.g., notenough lanes, a system widening is the only solution), but often, clearing unique bottlenecklocations within the system demonstrates that the uniform highway segments may not necessarily be underdesigned.Traditional capital solutions often grew from the misconception that a multilane facilityshould be designed to alleviate the recurring peak hours each day. The problem is thatfunding for these large scale projects is limited, and right-of-way is often restricted, suchthat these projects take a long time (many years) to complete. As a result, recurring congestion historically went untreated, or at least competed against other worthy projects,until funding became available to “catch up” to the problem that had grown from theday the facility opened.With a shift in the focus away from perceiving that recurring congestion is systemic (andthus treatable with only large projects), we must explore a wider-range of improvementstrategies that are possible in the short-term. While these will never replace the need forcorridor-wide fixes – especially at the “mega-bottlenecks” such as major freeway-to-freeway interchanges – bottleneck-specific improvements can provide congestion relief.“Bottlenecks can’t be fixed without massive reconstruction.”With the focus of transportation planners on major capital projects, it has been assumedfor many years that bottlenecks cannot be fixed without massive reconstruction of aninterchange or corridor. There are numerous examples where agencies opted to makelower-cost improvements that made significant improvement 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 specialevents can happen at any time, congestion is not restricted to peak times of the day. Theimprovements made at bottlenecks primarily to address peak-period problems will carryover to the times outside of the peak when congestion occurs.

Demand-Related

Systemic High Demand
9

accelerating the development of multistate, multiuse transportation corridors byrunning a competition to select three to five major growth corridors in need oflong-term investment.Target Major Freight Bottlenecks and Expand Freight Policy Outreach.The Department is working to find and implement solutions to freight transportation and border congestion that will facilitate trade and travel without compromising either highway safety or the vital mission of securing America’s borders. This area of interest emphasizes a Southern California Freight Outreacheffort that will broker consensus on immediate and longer-term transportationsolutions by bringing together key stakeholders.Accelerate Major Aviation Capacity Projects and Provide a Future FundingFramework. The Department is working to address congestion in the aviationsystem by designing and deploying the Next Generation Air TransportationSystem. In addition, the Department will advance reforms that lead to bettermanagement of airport and airspace congestion.

ttleneck locations and the

 congestion is prevalent,

g, and analyzing bottle

 can mathematically iden

ngestion or poor level of

a to identify where and

Historical data is used to

mplished with a range 

pected to be bottlenecks

ls can provide a detailed

can assist in determining

ottleneck analysis, care

n doesn’t just transfer the

may be “metering” flow

s acceptably, but the

ttleneck.

mprovements are inclusive

ongly considered.

es, the queue formed by a

tream of it. Improving

n locations. It is impor

dden bottlenecks” exist at

s are included. There are

f geometrics and abrupt

yzed by analysis tools.

ed to identify those prob

oblems and potential solu

k improvements can be

funding is secured.
17

U.S. DOT) announced the

ransportation Network

itiative is focused on

estion. Because of this

ucing congestion, not

Each area includes activi

short term and to build

duction efforts.

er into Urban Partnership

, bold, and innovative

 important to note that

mprehensive package that

 alternatives, such asnsportation; ensuring that

oper technology is in

he pricing strategy.

Department is working

nt in the construction,

re.

ents. The Department is

cal improvements aimed

ng state efforts to 1) pro

incident management

ms and quick clearance

mprove work zone safety

w-cost construction

Short-Term, Low-Cost Improvements Here is a sampling of operational remediations.

1. Using a short section of shoulder as an additional travel lane.2. Re-striping merge or diverge areas to better serve demand.3. Reducing lane widths to add a travel and/or auxiliary lane 

(e.g., re-striping).

4. Modifying weaving (e.g., adding collector/distributor or through lanes).5. Metering or closing entrance ramps.

Bottleneck

Types

Mitigation Measures

Auxiliary

Lanes

CollectorDistributor

Road

Paved Right

Shoulder

Paved Left

Shoulder

Shoulder/

Plus Lane

Re-Stripping

to Add More

Narrow Lanes

All Purpose

Lane

(Concurrent

or Reversible)

HOV Lanes

(Concurrent

or Reversible)

Truck

Reversible

Ramp

Metering

Temporary

Ramp

Closures

Traffic

Diversion

Information

Heavy OnRamp Demand ++ - + - ++ + ++ - + ++ - +WeavingSections + + + + + ++ ++ ++ + - - +Lane Drops ++ - ++ - ++ ++ ++ ++ + + + +Tunnels andBridges - - - - - ++ - + ++ - - +Horizontal andVertical Curves ++ - ++ ++ ++ - ++ + ++ + + +Narrow Lanesand LateralObstruction

+ + + + + - + + ++ + + +

InadequateAcceleratedand/orDeceleratedLanes

++ ++ ++ - ++ + + + ++ + ++ ++

Table 3. Mapping Bottleneck Problems to Mitigation Measures

++ = good solution + = may be helpful - = not applicable

Source: Adapted from interim materials from NCHRP Project 3-83.
7

may be addressed through increased expenditures. Environmental concerns may bemitigated and physical challenges overcome (for example, through tunneling).However, the resources to fund such improvements simply are not availablethrough traditional sources. For this reason, many professionals in the transportation community are enthusiastic about the opportunities potentially afforded bypublic-private partnerships and road pricing.Improve the Management and Operation of the System. This area of interest involves better managing the vehicles that are actually on the road, and the roaditself. “Smart” roads, traveler information, and improvements to the managementand operation of the facility are options available for using the available systemmore productively and bringing it to peak performance. Management and operations strategies are targeted at managing temporary disruptions (e.g., incidents) in away that will return the system to full capacity quicker; ensuring more efficient dayto-day operations through coordinated and up-to-date traffic signal timing and operational improvements to relieve bottlenecks; and providing real-time informationabout the system so that travelers can make immediate decisions about when, where,and how to travel, and transportation agencies can make real-time adjustments toimprove system operations.Effective and efficient management and operation of the system is foundationalto all of the above congestion reduction strategies. This is true because as trafficvolumes have grown over time and physical capacity has remained relatively constant, the system has become less able to absorb “surprise” – or nonrecurringevents. In the realm of managing the highway system, the margin for error is verysmall and continues to decline. In addition, operational fixes to the system arealso helpful in addressing the recurring congestion resulting from bottlenecks andimproper traffic signal timing. This is particularly significant in view of the factthat bottlenecks account for 40 percent of congestion and are often difficult toresolve via major reconstruction.

How Are Freight Delays 

Costly to You?

• Congestion means longer travel

times and increased costs inwasted fuel and driver remuneration. To compensate, companies add vehicles, hire moredrivers and employees, andextend their hours to accommodate us, eventually passingthrough these costs to shippersand consumers.

• FHWA estimates these

increases (to the company)to be between $25 and $200per hour depending on theproduct carried.

tion thresholds are

impending. This mostly

ave” that results through

 to bottlenecks and choke

.

r and smooth merges. A

 be 20th to merge into the

jumpers that bull ahead,

nes until they force their

quires on-site enforce

now the process and are

ement principles.

es.

mand into alignment. As

ons are relaxed, pricing

actitioners as a powerful

 address bottleneck 

Cooperative Highway

nts for Recurring Freeway

transportation personnel

me the low-cost improve

ns in their jurisdictions.

s showed that agencies

ks, most of them low

ategies include making

ctive additions to it.
19

ly assessed electronically

n facilities. Road-use

vide incentives to shift

alternative modes; or 

ther trips or simply to 

rst, it seeks to balance

space. Second, it seeks

aintaining, and expanding

nd. Third, it seeks to

bjective may include rev

nd If to Travel. The goal

 a given road. This may

s such as employee

e attractive to use. Also

grams that, for example,

y. Although there is sig

city, concerns about air

ay of expanding the sys

normously expensive and

ew construction that

er to preserve or improve

or implemented by the interviewed agencies were:

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

In general, the interview responses to this question suggest there is a trend towardsfavoring the implementation of ramp metering as a low-cost bottleneck improvement. Such systems are particularly attractive since they allow traffic managers to directly control freeway demand levels. This unique capability providesa valuable tool for managing freeway bottlenecks that does not involve large andexpensive capacity expansions.Some of the key questions and considerations in the development of improvementalternatives for bottleneck removal include:

• Is there an inside shoulder that would create a usable traffic lane for a short

section of freeway?

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

while allowing adequate clearance to barriers (2 feet) and an outside shoulder? If not, are they short enough that a loss of shoulder as a breakdownlane 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 mustbe resolved?

• Are vertical clearance issues, grade-matching, and sight distance problems created?• If a shoulder is considered for conversion, is there right-of-way (ROW) to

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

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

traffic which is affected by it be better accommodated?

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

breakdown again or is this truly the clearing up of a “kink” in the system,without being a capacity addition which will overload some other part ofthe facility?1

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

Freeway Bottleneck Analysis Methodology.
5

Consistent with the paradigm of its causes, there are four distinct, but nonethelessrelated strategies available to attack congestion. Table 1 outlines the four strategies and provides examples of specific options for furthering each of them. It isimportant to note that the strategies, while individually of merit, work best whenimplemented as a coordinated package of tools.Bring Supply and Demand into Alignment through Congestion Pricing.Congestion pricing or peak-period pricing, entails fees or tolls for road use thatvary by level of vehicle demand on the facility. As with market pricing in othersectors, road pricing helps allocate limited supply – in this case that of availableroad space.

Please Dispose of 

Gasoline Properly!

Imagine purchasing from 8 gallonsof fuel (in a small urban area) to36 gallons of fuel (in a largeurban area) per year, and thenthrowing it away. In fact, that isone estimate of how much fuel iswasted while idling in congestionper year. At $3.00/gallon, thatamounts to $24 to $108 “out ofpocket” for every driver in theseareas. The average was 28 gallons, or $84. 

2005 Urban Mobility Study, The Texas

Transportation Institute (TTI), 2005

Bring Supply and Demand into Alignment through Congestion PricingRoad Pricing Ramp MeteringCorridor ManagementProvide Real-Time Travel InformationProvide Better Choices as to How,When, Where, and If to TravelProvide More Attractive Alternatives to Single Occupant Vehicle Transportation (Including Better Transit, More Telecommuting, or High Occupancy Toll Lanes)Provide Real-Time Travel InformationStrategically Invest in New Transportation CapacityAdd New Construction on New AlignmentImprove the Management and Operation of the SystemQuickly Restore Capacity After Traffic Disrupting Events

– Improve the Management of Traffic Incidents– Improve Mobility at Work Zones– Respond Effectively to Inclement Weather Conditions– Plan Ahead for Special Events

Improve the Day-to-Day Operation of the System

– Improved Traffic Signal Timing– Operational and Low-Cost Construction Improvements to Relieve Bottlenecks (e.g., restriping)

Provide Real-Time Travel Information to Agencies and System Users

Table 1. Examples of Strategies to Reduce Highway Congestion

ost treatments can provide

of improvements have

e emerging. This section

sing these strategies to

ction programs;

ton State;

a;

k;

Atlanta Downtown 

ms in the Puget Sound

d system for two

 meters allow the freeway

vehicles has resulted in

t ramp from I-405 to south

d the traffic volumes han

he addition of a “weaving

stinations. Where traffic pat

o be relieved and provide a

national port business.

peeds. An additional lane
21

this capacity. Congestion

ither “recurring” or 

community of trans

mitigate or reduce conges

 time and place on the

 is simply not adequate 

other hand, nonrecurring

c incidents, and bad

cur on an already con

nts a pie chart showing

Maryland’s Quick Fix at Interstate-70/Intestate-695A recent project at the I-70/I-695 (Baltimore Beltway) interchange outside of Baltimore was discussed. The eastbound approachfrom I-70 to I-695 backed up on to the mainline of I-70, restricting through traffic. Widening the entire ramp would have beenvery expensive due to the need for major bridge reconstruction. Instead the ramp was widened up to the bridge. This providedadequate storage to relieve the backup on to the mainline and did so at a reasonable cost.

Inadequate

Physical

Capacity

(Bottlenecks)

40%

Traffic Incidents 

25%

htm.

The interchange of I-75 and Bruce B. Downs Boulevard in suburban Tampa exhibited the characteristics of a bottleneckamenable to a low-cost improvement. One of the major traffic flows is a right turn from the ramp and a quick left turn onto anarterial. The weaving on the cross street caused queuing on the ramp which often backed up to the freeway mainline. The problem was addressed by adding a free right turn lane and a signalized right turn lane. The traffic that needed to make the quick leftturn is signed to use the signalized right turn lane. Queuing on the mainline is no longer a problem.

TxDOT District Freeway(s) And Limits Description of Bottleneck Improvement(s)

Dallas EB IH 30, IH 35E to IH 45 Changed exit ramp to Harwood to become entrance ramp from the 

collector-distributor road.

Fort Worth NB SH 360 @ Division (SH180) Converted outside shoulder to auxiliary lane between two closely spaced

exit ramps.

Dallas NB IH 35E, IH 30 to Dallas

North Tollway

Addition of two auxiliary lanes by inside shoulder conversion.

El Paso EB IH 10 @ U.S. 54 Restriped one-lane ramp to two lanes, dropped main lane at exit, added at

entrance, added auxiliary lane.

Dallas EB SH 190 to SB U.S. 750 Restriped to give entrance ramp from SH 190 its own lane onto U.S. 75.Dallas NB IH 35E Ramp to Dallas North

Tollway 

Restriped merge to allow ramp its own lane onto approach to tollbooths.

Dallas NB-SB IH 35E, LP 12 to IH 635 Converted inside shoulders to travel lanes for 3.0 miles and removed two

inside merges.

Dallas WB IH 30 Ramp to SB IH 35E Restriped to balance freeway capacity with freeway volumes at merge.Fort Worth EB IH 20 to NB SH 360 Added deceleration lane to IH 20 before exit; dropped main lane at exit,

added back at SH 360 entrance ramp.

Fort Worth SB SH 360 to WB IH 20 Auxiliary lanes on SH 360, dropped main lane on IH 20 at SH 360 exit,

added lane at SH 360 entrance.

Fort Worth SB SH 360 @ Division Closed entrance ramp, forcing traffic through signal, added 

auxiliary lane to next entrance.

Dallas EB IH 635 to NB U.S. 75 Widened and restriped left-side ramp from one to two lanes.Dallas SB U.S. 75 to WB IH 635 Converted inside shoulder on IH 635 to allow 

ramp from U.S. 75 its own lane.

Low-Cost Bottleneck Improvements in Texas. The Texas Department of Transportation (TxDOT) has undertaken significant low-cost freeway bottleneck improvements in recent years. These include:

Source: Walters, Carol H, Cooner, Scott A., and Ranft, Stephen E., Looking Again at Bottlenecks on Freeways Evaluating Case Studies in Texas, 

November 15, 2004, http://pubsindex.trb.org/document/view/default.asp?lbid=775977.
3

• Costs to passengers of having to leave early for a destination because of

congestion-related unreliability in travel times.

Not only does congestion dampen the economy, it also impacts the way we live:Parents miss events with their children; friends and families find it harder to spendtime together; and civic participation is increasingly difficult. Evidence suggeststhat each additional 10 minutes in commuting time cuts involvement in community affairs by 10 percent.The root causes of congestion have long been understood, and there is nowbroad consensus that congestion generally reflects a fundamental imbalance ofsupply and demand. That is, during hours of peak usage of the transportationfacilities most desirable to motorists, the supply of, for example, roadway capacityis insufficient to meet the demand for those facilities. Economists have longunderstood that such an imbalance stems from inefficient pricing, where thetrue costs of usage are not reflected in prices paid by the users. For example,travelers are not generally charged for the impact their trip will have on othersusing the same facility (e.g., increased levels of congestion) or on other membersof society (e.g., increased air pollution). In fact, in this country, access to highwaytravel is, for the most part, rationed by delay.The imbalance of supply and demand leading to congestion is also impacted bythe absolute volume of traffic (e.g., demand) on a given facility relative to its physical capacity (e.g., supply). When we look at traffic congestion from a demandperspective, we are looking at how many vehicles compete for space on a particular facility at a given time. The demand for a facility is a function of individualdecisions as to when, where, how, and even if highway travel will take place.

ds, we will take the oppor

t one of several program

re information may be

Washington Post, 

June 2007

A June 2007 article in theWashington Post laments the difficulty parents in outlying suburbshave in attending their kids’evening soccer, t-ball, baseball,and softball games. Parents whoare coaches, and have responsibility to be early to set up the field,have an especially hard time,often arriving in their officeclothes directly from work.Games are routinely pushed back30 to 60 minutes; leagues areover taxed because only one, andnot two, games can be played perevening on the same field. 

n the past few years. The

he I-75/I-85 merge just

ntified as the nation’s sixth

ottleneck study.

o add ramp storage and

d and Ellis Street south

ramp meters in that sec

d Avenue). The ramp

ent in time for the four

ngestion hours was

Locally Known as the

-Period

om Memorial Drive.
mension – duration,

by traffic congestion has

etch from 2 or 3 hours in

on people. Larger areas

cent of travel. This sta

Many cities have a few

raffic. Weekend traffic

r major shopping centers

delay penalty (the extra

om 13 to 37 percent in the

37 percent more travel

percent 10 years earlier.

g trips, doctor visits, and

both monetary and socie

 Texas Transportation

ed to be $63 billion based

f wasted fuel calculated in

 2003 was equivalent to

reover, these congestion

e the growth rate of the

d to rise to 1.6 percent of

ss than half of the overall

lude:

ies outside of the 85-city

es and labor market sizes;

23

Active bottleneck – When traffic flow through the bottleneck is not (further) affected bydownstream restrictions.Auxiliary lanes – Typically, any lane whose primary function is not simply to carry throughtraffic. This can range from turn lanes, ramps, and other single-purpose lanes, or it can bebroadened to imply that a traffic-bearing shoulder can be opened in peak-periods to help alleviate a bottleneck, and then “shut back off” when the peak is over.Bottleneck – There can be many definitions. Here are a few that are typically used.1) A critical point of traffic congestion evidenced by queues upstream and freeflowing traffic downstream; 2) A location on a highway where there is loss ofphysical capacity, surges in demand (traffic volumes), or both; 3) A point wheretraffic demand exceeds the normal capacity; and 4) A location where demand forusage of a highway section periodically exceeds the section’s physical ability tohandle it, and is independent of traffic-disrupting events that can occur on theroadway.Capacity – The maximum amount of traffic capable of being handled by a given highwaysection. Traffic engineers usually speak in terms of “free flow” capacity.Congestion – The FHWA “Traffic Congestion Reliability” reports define congestion as anexcess of vehicles on a portion of roadway at a particular time resulting in speeds that are slower – sometimesmuch slower – than normal or free flow speeds. (Congestion is) stop-and-go traffic. Previous work has shownthat congestion is the result of seven root causes2 often interacting with one another. Since a bottleneck is a

cause of congestion, congestion cannot be solely analogous to a bottleneck. Congestion ismore. For example, a “congested” corridor may harbor multiple bottlenecks or any combination of the seven root causes.Downstream traffic – Traffic that is beyond (past) the subject point on a highway.Hidden bottleneck – A highway location where some type of physical restriction is present,but traffic flow into this area is metered by an upstream bottleneck so the location does notappear as a bottleneck under prevailing conditions. Removal of the upstream bottleneck willcause the hidden one to emerge as a new bottleneck.

2 The seven root causes are bottlenecks (a.k.a. “capacity constraints”), incidents, work zones, weather, poorly timed signals 

et.al., special events, and over-capacity demand (i.e., daily and seasonal peaks superimposed on a system with a fixed capacity).Some sources cite only six root causes because they see over-demand as an inherent subelement necessary for any of theother causes to exist in the first place. Put another way, absent over-demand there would just be “volume,” but not necessarily“congested” volume.
1

The delays arising from traffic congestion are a fact of life in many communities.According to a February 2007 Harris Poll, just over one-third (37 percent) cite traffic congestion as a serious problem in the community, while one-quarter say trafficcongestion is a serious problem that is not being addressed. Close to half of allcongestion happens day after day at the same time and location. Much of thisrecurring congestion is due to physical bottlenecks – specific points on the highwaysystem where traffic flow is restricted.While many of the Nation’s bottlenecks can best be addressed through costly majorconstruction projects, there is also significant opportunity for the application ofoperational and low-cost infrastructure solutions to bring about relief in the shortterm. This document, Traffic Bottlenecks: A Primer – Focus on Low-Cost Operational

Improvements, describes bottlenecks 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. While updated hardcopy versions will occasionally be made available, an electronic version will be regularly supplemented as new strategies are identified from within the transportationcommunity. The Primer is a key resource for Federal Highway Administration’sLocalized Bottleneck Reduction (LBR) Program, providing a virtual forum for peerexchange between members of the transportation community interested in alleviating bottleneck congestion. The LBR program, initiated in 2007, is designed toexpand the portfolio of bottleneck reduction tools available to transportation agencies to encompass innovative, readily adopted strategies for reducing congestion atbottleneck locations.

ne, or other atypical event.

way via use of control devices

of vehicles entering or leaving

nt of ramp metering is to

ough vehicles. Done proper

ons.

a traffic condition (i.e., a bot

on and at the same time daily,

peak-hour slowdowns at

ese events because one

he same manner and place.

ols that rely on input of traf

parameters of each input, in

ngineers can test different

t point on a highway.

U.S. Department of TransportationFederal Highway Administration