Slugging in Houstin

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Slugging in Houston

23

Slugging in Houston—Casual

Carpool Passenger Characteristics

Mark W. Burris, Texas A&M University

Justin R. Winn, Wilbur Smith Associates

Abstract

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

passengers.

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.

Introduction

As congestion has worsened in our nation’s metropolitan areas, transportation

professionals have explored various methods to increase the effective use of the

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Journal of Public Transportation, Vol. 9, No. 5, 2006

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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 (http://www.slug-lines.com) 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.

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Slugging in Houston

25

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

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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.

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Slugging in Houston

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Data

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

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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

holidays).

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

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Slugging in Houston

29

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.

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Journal of Public Transportation, Vol. 9, No. 5, 2006

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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.

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Table 2. Descriptive Statistics of Surveyed Travelers

1

Significant (p ≤ 0.05) difference when comparing all four modes.

2

Significant (p ≤ 0.05) difference when comparing casual carpooling and transit.

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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.

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Slugging in Houston

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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

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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.

Methodology

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.

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Table 4. Model Coefficient Estimation Results

(Casual Carpooling vs. Transit)

1

1

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

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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

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Table 6. Model Coefficient Estimation Results (All Four Modes)

1

Base alternative is driving alone on main lanes with utility of zero.

*Significant at the 95 percent confidence level.

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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.

Summary

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.

Acknowledgments

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-

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tributions of numerous individuals and organizations that made the successful

completion of this article possible.

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About the Authors

Mark W. Burris

(mburris@tamu.edu) 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

(jwinn@wilbursmith.com) 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.