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Monday, March 10, 2008

Slugging in Houstin

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Slugging in Houston
Slugging in Houston—Casual
Carpool Passenger Characteristics
Mark W. Burris, Texas A&M University
Justin R. Winn, Wilbur Smith Associates
In the last 30 years, determined travelers have developed a new method of travel
that offers the benefits of traveling on an HOV lane without forming traditional
carpools. Casual carpools, also known as “slugging,” are impromptu carpools formed
among strangers to meet the occupancy requirements of HOV lanes. In this research,
survey respondent data from Houston, Texas, were used to examine casual carpool
Results of the analyses revealed that being on a commute trip, making more trips per
week, being between the ages of 25 and 34, and having professional/managerial or
administrative/clerical occupations all increased the likelihood of a traveler choosing
to casual carpool. Additionally, having a household income between $25,000 and
$35,000 significantly reduced the likelihood of casual carpooling.
Understanding the types of travelers who casual carpooled and the information
gleaned in these analyses can be used to better evaluate HOV and HOT lane use
and performance. Casual carpool passengers can comprise a significant portion of
HOV/HOT lane person movement and should be considered when investigating HOV
or HOT lane implementation.
As congestion has worsened in our nation’s metropolitan areas, transportation
professionals have explored various methods to increase the effective use of the
Journal of Public Transportation, Vol. 9, No. 5, 2006
transportation infrastructure. One such method is the implementation of high-
occupancy vehicle (HOV) lanes. HOV lanes are typically built on congested free-
ways, and allow vehicles that meet specified occupancy requirements to bypass
delays associated with driving alone on congested general-purpose lanes (GPLs)
of the freeway. HOV lanes encourage carpooling and increase person movement
along congested corridors (Turnbull 2004). HOV lanes promote the increase of
person movement through higher vehicle occupancies by providing travel time
savings to carpoolers (Turnbull 2004).
In the last 30 years, determined travelers have developed a new method of travel
that allows them to receive the benefits of traveling on the HOV lane without
forming traditional carpools (LeBlanc 1999). This new mode, known as casual car-
pooling or “slugging,” consists of impromptu carpools formed among strangers to
meet the occupancy requirements of HOV lanes.
The process of forming a casual carpool is relatively simple. Casual carpool pas-
sengers typically meet in a public area that has ample available parking, nearby
public transit as an alternate mode in case a casual carpool is unavailable, and close
proximity to the HOV facility. Drivers (also known as “body snatchers”) arrive and
pick up enough passengers to meet the HOV lane eligibility requirements. Driv-
ers then travel along the HOV lane and drop off passengers in a public location,
typically in the downtown area of a city. Details of the casual carpool process vary
slightly depending on location.
Currently, organized casual carpooling occurs in three U.S. metropolitan areas:
Washington, D.C. (LeBlanc 1999; Reno, Gellert, and Verzosa 1989; Spielberg and
Shapiro 2000), San Francisco (Beroldo 1990; RIDES for Bay Area Commuters, Inc.
1999), and Houston (Ojah and Burris 2004).
The slugging system in Washington, D.C., which has existed for more than 30 years,
is well organized with a large number of pick-up and drop-off locations and a
website ( for local slugs and bodysnatchers. Conversely,
slugging in the San Francisco Bay area occurs in one general area. Passengers and
drivers meet in the morning peak period to form carpools and cross the Bay Bridge.
Passengers are usually dropped off in the downtown area and typically use transit
for their return trips. Casual carpooling in Houston occurs at three locations, all
of which feed the downtown area. Passengers meet at park-and-ride locations on
I-10 and US 290 that have direct access to HOV lanes. Drivers arrive throughout
the morning and pick up the necessary number of passengers to meet the HOV
occupancy requirement.
Slugging in Houston
An important similarity among these three locations is that the HOV lanes require
three or more occupants, whereas the vast majority of HOV lanes in the United
States allow vehicles with two or more occupants. This higher occupancy require-
ment plays a significant role in the formation of casual carpools. At the same time,
urban freeways are becoming increasingly congested, encouraging more travel-
ers to use HOV lanes and, therefore, more HOV lanes will have to increase their
restrictions to three or more occupants. As more HOV lanes institute higher occu-
pancy restrictions, the need to understand the complex issue of casual carpooling
becomes exceedingly important.
Despite its presence for more than three decades, casual carpooling has yet to
expand beyond these three cities. Casual carpooling can increase person move-
ment along congested corridors and can provide substantial travel time savings
for users. However, it is not marketed or regulated in any way by transporta-
tion officials. As these carpools are formed among strangers, there are potential
liability issues that could surround agency support of casual carpooling. This does
not mean that the effects of casual carpooling and characteristics of its users are
not important to transportation engineers and planners. With the potential to
increase person movement and provide better HOV lane utilization, casual car-
pooling could represent a significant portion of daily HOV lane travelers, particu-
larly if (1) more HOV lanes restrict usage to vehicles with three or more occupants
and/or (2) future HOV facilities are constructed with casual carpoolers in mind.
This research took an in-depth look at casual carpooling in Houston, with empha-
sis on the travel time savings gained by those choosing this mode. Additionally,
survey data were examined to gain insight into the socioeconomic and commute
characteristics of Houston casual carpoolers and to generate mathematical mod-
els that further consider the socioeconomic and commute characteristics that
indicate a higher likelihood of a traveler choosing to casual carpool.
Casual Carpooling in Houston, Texas
The casual carpooling phenomenon appears to have begun more recently in
Houston than in Washington or San Francisco. Although no documented evi-
dence exists to pinpoint when casual carpooling began in Houston, newspaper
interviews of casual carpool users indicate that the mode has been used since 1990
(Wall 2002).
Casual carpooling in Houston occurs in three locations: Kingsland Park-and-Ride
lot, Addicks Park-and-Ride lot, and Northwest Station Park-and-Ride lot. The
Journal of Public Transportation, Vol. 9, No. 5, 2006
Kingsland and Addicks lots are located on I-10 (Katy Freeway) west of downtown
Houston; the Northwest Station lot is located on US 290 (Northwest Freeway)
northwest of downtown Houston. Each park-and-ride facility is used primarily for
transit and offers direct-connect ramps to a barrier-separated HOV lane. Casual
carpool passengers form a line near transit pick-up locations and wait for drivers.
Drivers arrive periodically and pick up enough passengers to meet the HOV lane
occupancy requirement. If passengers are unable to join a casual carpool, they
have the option of using transit, which runs throughout the day from the park-
and-ride facilities. Most casual carpools form between 6 A.M. and 9 A.M. (Ojah
and Burris 2004). As bus headways increase significantly after 9 A.M. and most
commuters have already traveled to work, the use of casual carpools decreases
significantly, dropping to near zero.
Casual carpooling in Houston occurs exclusively on the city’s two high occupancy/
toll (HOT) lanes (the only two HOV lanes that restrict usage to three or more
occupants during part of the day). The vehicle occupancy requirement on I-10
and US 290 is HOV2+ for most of the day, but, due to congestion, it was raised to
HOV3+ from 6:45 A.M. to 8 A.M. and 5 P.M. to 6 P.M. on I-10 and from 6:45 A.M. to
8:00 A.M. on US 290. The lanes are closed temporarily during the middle of the day
for direction reversal. During the HOV3+ periods, HOV2 vehicles may enter the
lane by paying a $2 toll. This program was first implemented on the Katy Freeway
HOV lane in 1998 and was expanded to include the Northwest Freeway HOV lane
in 2000. Participants were required to open an account, mount a transponder and
hangtag on their vehicle, and pay a $2.50 monthly service charge. The behavior
of casual carpoolers would change during the restricted periods as drivers would
typically pick up only one passenger during the HOV2+ periods, but would pick
up two passengers during the HOV3+ period. The majority of casual carpooling
occurs during the HOV3+ period (see Table 1 on p.29). A separate survey of driv-
ers who paid the $2 toll to travel in the lane during peak periods revealed very few
(7%) pick up a single slug (Burris and Appiah 2004). This was not surprising as the
cost (extra time) spent picking up the second slug was relatively small compared
to the $2 toll.
Slugging in Houston
The analysis of casual carpool passenger behavior required socioeconomic and
commute characteristics of casual carpool passengers. Most of the necessary data
were collected by the Texas Transportation Institute through a survey distributed
to casual carpool passengers as part of a larger traveler survey in November 2003
(Burris and Stockton 2004). However, additional data on corridor travel speeds
and carpool headways were collected to estimate the time savings benefit gained
by casual carpoolers.
Based on video license plate data, surveys were mailed to drivers using the general
purpose and HOV lanes during both peak and off-peak traffic periods. Each survey
was designed specifically for the group to which it would be distributed (HOV lane
during peak periods, main lane off-peak, etc.). Additionally, surveys were produced
for transit users and casual carpool passengers. However, rather than being mailed,
the transit passenger surveys were conducted on-board the buses, and casual car-
poolers were handed surveys while they waited for a ride. All surveys had questions
regarding trip purpose, time of day, and socioeconomic characteristics. A set of
questions specific to casual carpooling was also included. A series of stated pref-
erence questions that asked respondents to identify their preferred travel mode
given specific travel time and fee (toll) options was included in all surveys.
A total of 539 questionnaires were distributed to casual carpool passengers at the
three park-and-ride facilities in Houston. Of the 539 surveys, 216 were returned
for a total response rate of approximately 40 percent. On the day the surveys were
handed out, 7 percent of casual carpool passengers refused to take one, indicat-
ing an approximate total of 578 casual carpool passengers that day. This number
closely matched casual carpool passenger counts performed in June 2003. There-
fore, even though relatively little was known about the total number of casual
carpoolers in Houston, the 216 returned surveys were believed to be sufficient so
that the responses were representative of the group.
The final dataset used in the analysis excluded a number of the 216 responses.
For this analysis, only trips beginning between 6 A.M. and 9 A.M. (eliminating 8
respondents) were included to focus on the time period during which the vast
majority of casual carpooling occurred and when the primary alternative mode
(transit) had consistent headways. Additionally, for the calculation of descriptive
statistics and estimation of mode choice model coefficients, only respondents
who used casual carpooling at least three to four times per week were considered
to allow the analysis to focus on travelers who frequently casual carpooled. This
Journal of Public Transportation, Vol. 9, No. 5, 2006
further reduced the dataset by another 59 respondents, leaving 149 respondents
for the casual carpool analysis.
Travel Time Savings
The casual carpool passenger survey included questions regarding travel time sav-
ings, which provided travelers’ perceived travel time savings on the HOV lane. To
estimate the actual travel time savings gained by casual carpool passengers, travel
time data along the HOV lanes as well as the GPLs were required. TranStar, Hous-
ton’s traffic management center, recorded average speed data on the corridor.
This information was used to calculate the various travel times (Houston TranStar
Real Time Traffic Information). The data used in this analysis were average speeds
along the HOV and GPLs for the entire 2003 year (not including weekends and
To calculate travel time savings offered by casual carpooling, consideration was
made for the amount of time necessary to park at a carpool formation site and
wait to join a carpool. Parking and wait times at the formation site were manually
observed during a typical morning peak period. On Wednesday, June 30, 2004,
three data collectors observed parking and wait times at the Addicks Park-and-
Ride location on the Katy Freeway. One data collector observed people arriving
at the facility and measured the amount of time necessary to walk from their cars
to the casual carpool formation site. Forty-two persons were observed taking an
average of 105 seconds (± 7.6 seconds at a 95 percent confidence interval) to walk
from their cars to the site. Two other data collectors recorded the amount of time
that casual carpool passengers waited in the casual carpool line prior to entering
a vehicle. The 147 casual carpool passengers experienced an average wait time of
144 seconds (± 17.8 seconds at a 95 percent confidence interval). Combining the
walking and waiting times with the travel time savings indicated that casual car-
pool passengers could save as much as 13 minutes over driving alone on the GPLs
(see Table 1). Additionally, the number of casual carpool passengers was generally
higher during times of larger travel time savings.
In comparing carpooling and riding transit, it was necessary to determine the
approximate time spent waiting for a bus, as this wait time was the only travel
time difference between the two modes. Transit users and casual carpoolers spent
the same amount of time arriving at the park-and-ride lot and walking to the
queues. Casual carpool passengers and transit users incurred similar travel times
after being dropped off because carpool passengers were typically dropped off at
Slugging in Houston
or near bus stops. Additionally, the in-vehicle time for the two groups was similar
as these express buses only had 2 to 3 stops on their route (this includes the stop
where slugging occurs and the destination stop). Bus headways for each of the
three park-and-ride locations during the morning peak period were used to calcu-
late average wait times. The average headway was 10 minutes on the Katy Freeway
Table 1. Time Savings (in minutes) Gained by Casual Carpool Passengers
Compared to Driving Alone on the GPLs
Note: The travel time savings calculation assumed very conservative values for the amount of travel
time saved by casual carpoolers. For example, it was assumed access to the park-and-ride lot took
several extra minutes over just entering the freeway as an SOV. Most likely, casual carpoolers who
traveled when the estimated travel time savings was negative actually had positive travel time
savings, but they did not meet the conservative assumptions used. For example, their access to the
park-and-ride lot may have taken no extra time versus accessing the freeway as an SOV.
Journal of Public Transportation, Vol. 9, No. 5, 2006
and 8 minutes on the Northwest Freeway. The average time spent waiting for a
bus was assumed to be half of the average headway based on the assumption of
random arrivals of transit passengers (Meyer and Miller 2001). Casual carpoolers
saved an average of 2 minutes 36 seconds over transit on the Katy Freeway and 1
minute 36 seconds on the Northwest Freeway.
Other factors besides travel time savings might have influenced the mode choice
of the travelers. Monetary costs (e.g., transit fare, fuel) or trip purpose could have
affected a traveler’s decision (Wall 2002). Socioeconomic characteristics could also
have had a major influence on a traveler’s decision to casual carpool. Travelers may
have valued the reliability of travel times on the HOV lane. The survey data were
used to determine what, if any, trip and socioeconomic characteristics increased
the likelihood of a traveler choosing to casual carpool on a frequent (3 or more
times per week) basis.
Comparison of Traveler Characteristics by Mode
The survey data were initially examined for significant differences (p ≤ 0.05)
among four groups of travelers based on their primary mode choice: driving on
main lanes, using HOV lane with a traditional carpool, casual carpooling, and tran-
sit. A Chi-Square test assessed significant differences among the binary variables,
and a one-way analysis of variance (ANOVA) examined the continuous variables.
Additionally, a Kruskal-Wallis test determined any significant difference between
groups for the ordinal variables of age, income, and education.
The results of the statistical tests revealed significant differences among travelers
in the four primary morning modes of travel (Table 2). The percentage of respon-
dents on commute, recreation, school, and other trip types was significantly differ-
ent among the four groups. Casual carpoolers were more likely to be on commute
trips. The percentage of respondents ages 25 to 34 and 65+ was significantly differ-
ent among modes. A much higher percentage of casual carpoolers were between
ages 25 and 34. The average household size, percentage of single adult house-
holds and married without children households, and the number of vehicles per
household also differed among modes, with HOV users having significantly larger
households. A difference was also found for those with occupations that were pro-
fessional/managerial, sales, homemaker, self-employed, or retired. Income ranges
of $25,000 to $35,000, $50,000 to $75,000, $100,000 to $200,000, and $200,000 or
more were also different among the four mode choices.
Slugging in Houston
Table 2. Descriptive Statistics of Surveyed Travelers
Significant (p ≤ 0.05) difference when comparing all four modes.
Significant (p ≤ 0.05) difference when comparing casual carpooling and transit.
Journal of Public Transportation, Vol. 9, No. 5, 2006
Next, similar statistical tests were performed to determine significant differences
(p ≤ 0.05) between travelers using just two mode choices: casual carpooling and
transit. These mode choices were specifically examined due to their symbiotic
relationship and the similarity of the modes since travelers on both modes (1)
use park-and-ride lots, (2) have someone else drive, (3) travel on HOV lanes, and
(4) are dropped off relatively close to their work. Also, casual carpoolers are often
former transit users (Beroldo 1990), and in this study more than 90 percent still
used transit for some of their similar trips (Table 3).
The results of the statistical tests (Table 2) revealed several significant differ-
ences between casual carpoolers and transit riders. A higher percentage of casual
carpool passengers were on commute trips and between the ages of 25 and 34,
while a higher percentage of transit riders were between the ages of 55 and 64. A
significantly higher percentage of casual carpoolers had professional/managerial
occupations, while a significantly higher percentage of transit riders had house-
hold incomes between $25,000 and $34,999.
Casual Carpool Passenger Characteristics
The surveys distributed to casual carpool passengers contained a series of ques-
tions that were exclusive to that group. These questions addressed the nature
of each traveler’s casual carpooling trip and his or her previous experience using
the mode (Table 3). For this analysis only, both frequent and infrequent casual
carpoolers were examined. The results provided insight into the practice of casual
carpooling in Houston, including what modes were commonly used for return
trips and how frequently respondents joined a casual carpool.
Survey responses indicated that most casual carpool passengers (65.3%) had
never met their travel companions before. However, almost one third indicated
that they had traveled with them once or twice, indicating that a relatively small
community of people used the mode consistently. More than 75 percent of users
noted that they casual carpooled at least three times per week. Passengers also
cited saving money (62.8%) and slow bus service (52.6%) as the two primary rea-
sons for casual carpooling. They indicated that they often use the bus for similar
trips and for the evening return trip. They also noted that money is rarely given to
the driver as compensation, which is consistent with casual carpooling practices
elsewhere in the United States.
Slugging in Houston
Table 3. Casual Carpool Passenger Characteristics (n = 208)
Note: Some percentages sum to over 100 percent as respondents could choose multiple answers
for some questions
Journal of Public Transportation, Vol. 9, No. 5, 2006
Mode Choice Model Estimation
To better understand casual carpoolers and the factors that affect their mode
choice, discrete choice model coefficients were estimated for two sets of choices.
The choice between casual carpooling and transit was evaluated with the first
model. The second model examined traveler choice of four modes: casual carpool,
transit, traditional carpool, and driving on GPLs.
Both models were estimated as discrete choice models. Discrete choice models
assume that each traveler makes his or her decision based on the utility of each
mode (Ben-Akiva and Lerman 1985). The traveler’s ultimate decision will deter-
mined by both the systematic utility based on measured variables and the random
utility of each mode. The model in this analysis was estimated using a logit model,
which assumes that random utilities follow an extreme value distribution (Small
and Winston 1999).
Casual Carpool versus Transit Mode Choice Model
Although many variables were tested when estimating the model coefficients, only
those variables significant at the 95 percent confidence level and not correlated
to other variables were left in the final model. The results of the discrete choice
model are shown in Table 4. For this model, the null choice was casual carpooling.
The utility function derived in the model describes the utility of the transit mode
relative to the casual carpooling mode that had all coefficients equal to zero.
The results of the model highlight some of the factors that describe selected types
of travelers who choose to casual carpool rather than use transit. The constant
coefficient is positive, indicating that all else being equal, travelers were more likely
to choose transit than casual carpooling. This was not surprising as many more
travelers used transit than casual carpools. The results also indicated that having
an income between $25,000 and $35,000 increased the traveler’s likelihood to use
transit rather than casual carpooling. However, being on a commute trip, making
a higher number of total trips per week, and/or being between the ages of 25 and
34 increased the traveler’s likelihood of forming casual carpools.
Slugging in Houston
Table 4. Model Coefficient Estimation Results
(Casual Carpooling vs. Transit)
Base alternative is casual carpooling with utility of zero.
*Significant at the 95 percent confidence level.
Four-Option Mode Choice Model
Several sets of variables were used for testing the four-choice model, using the
main lanes option as the null choice. Only variables significant at the 95 percent
confidence level remained in the final model. The variables used in the model as
well as which mode choice utility functions they were associated with are listed in
Table 5, while the model estimation results are shown in Table 6.
The constants for the HOV, casual carpool, and transit modes were all negative,
indicating that all else being equal, travelers were most likely to drive on the main
lanes. The trip purpose, age, and occupation (professional) variables applied only
to the casual carpooling utility function and indicated a number of factors influ-
enced casual carpoolers’ decisions. The coefficient for the trip purpose was positive,
indicating that being on a commute trip increased the likelihood that a traveler
Journal of Public Transportation, Vol. 9, No. 5, 2006
Table 5. Definitions of Variables Used in Logit Model (All Four Modes)
would choose casual carpooling over the other three modes, which duplicates the
results of the previous model. Professional/managerial or administrative/clerical
occupations also increased a traveler’s likelihood to use casual carpooling over the
other three modes. Thus, travelers with weekday jobs with typical workday hours
were more likely to casual carpool. This was not surprising considering the times
during which casual carpooling occurs. Travelers with typical workdays would be
more likely to encounter peak-period congestion if they drove alone on the GPLs.
The results also indicated being between the ages of 55 and 64 reduced a traveler’s
likelihood of casual carpooling, which reflected a possible increased willingness
among younger persons to try a newer, less-utilized mode of transportation.
In addition, having an income between $25,000 and $35,000 reduced a traveler’s
likelihood of casual carpooling, which was surprising considering the relatively low
expense of that mode. One possible explanation was that low-income persons
already used transit for many of their other trips, and they chose to use transit
Slugging in Houston
Table 6. Model Coefficient Estimation Results (All Four Modes)
Base alternative is driving alone on main lanes with utility of zero.
*Significant at the 95 percent confidence level.
Journal of Public Transportation, Vol. 9, No. 5, 2006
during the times of casual carpooling as well. Another possible explanation was
subsidized transit passes were available to low-income travelers. Travelers with
subsidized transit passes would have little to no money-savings incentive to casual
carpool. Also, the descriptive statistics indicated that travelers with incomes
between $25,000 and $35,000 were less likely to make commute trips, leading to
less use of casual carpooling because commuting is a primary factor that influ-
ences casual carpool use.
This research effort examined the use of casual carpooling in Houston, Texas. Sur-
vey results revealed that most casual carpool passengers often used transit for eve-
ning return trips and similar morning trips. Approximately 63 percent used casual
carpooling to save money and about 53 percent used casual carpooling because of
slow bus service. Most casual carpoolers (76%) used this mode three or more times
per week. Casual carpool passengers were significantly more likely to be on com-
mute trips and be between the ages of 25 and 34 (younger), but were significantly
less likely to have household incomes between $25,000 and $35,000.
The results obtained in these analyses provided some information on the charac-
teristics of travelers who chose to casual carpool. This information can be used to
better evaluate HOV/HOT lane use and future lane development considerations.
Casual carpooling has grown in popularity and should be considered when assess-
ing potential corridor improvements. Although potential liability concerns would
likely prevent agencies from actively promoting casual carpooling, they could
encourage it passively by constructing park-and-ride HOV facilities that are con-
ducive to the mode. Casual carpooling has the potential to improve the operation
efficiency of HOV/HOT facilities by improving person movement. Although there
are potential liability concerns, it may eventually become beneficial to promote
casual carpooling as a viable mode alternative.
This article reflects the views of the authors, who are responsible for the facts and
the accuracy of the data presented herein. This article was a result of research
conducted in cooperation with the Federal Highway Administration (FHWA),
the Texas Department of Transportation (TxDOT), and the Metropolitan Transit
Authority of Harris County, Texas. The authors gratefully acknowledge the con-
Slugging in Houston
tributions of numerous individuals and organizations that made the successful
completion of this article possible.
Ben-Akiva, M., and S. Lerman. 1985. Discrete choice analysis: Theory and application
to travel demand. Cambridge, MA: MIT Press.
Beroldo, S. 1990. Casual carpooling in the San Francisco Bay Area. Transportation
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Turnbull, K. F. 2004. History of HOVs. Committee on High Occupancy Vehicles Sys-
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October 18, 2004.
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About the Authors
Mark W. Burris
( specializes in congestion (or value)
pricing and has been involved in many projects in this area. He has served in an
evaluation and monitoring role for the Houston Value Pricing Project, Lee County
Variable Pricing Pilot Project, Queue Jump Value Pricing Project, Fort Myers
Beach Cordon Toll Study, and Traveler Credit Based Pricing. Prior to joining Texas
A&M University in August 2001, he was a senior research associate at the Center
for Urban Transportation Research (CUTR) at the University of South Florida in
Tampa. He is a member of the American Society of Civil Engineers, the Institute
of Transportation Engineers, and the Transportation Research Board’s Economics
Committee and Congestion Pricing Committee.
Justin R. Winn
( has worked as a transportation ana-
lyst in the Transportation Finance and Technology division of Wilbur Smith Asso-
ciates in Dallas, Texas, since January 2005. He was previously a graduate research
assistant with the Texas Transportation Institute at Texas A&M University in
College Station where he earned a B.S. and M.S in civil engineering. Mr. Winn is a
member of the Institute of Transportation Engineers and the International Bridge,
Tunnel and Turnpike Association.

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