Rideshare History & Statistics

Posted by admin on January 24th, 2009

This section of the site presents background information on ridesharing focusing largely on the US experience, with some international data presented. A brief history of ridesharing is presented along with historical trends and current statistics. The document concludes with a brief discussion of the quantification of future rideshare benefits, or rather the lack thereof.

Table of Contents:

  1. Selective History of Ridesharing
  2. Defining Ridesharing
  3. Historical Trend – Journey to Work
  4. Price of Gasoline & Disposable Personal Income
  5. Mode Share by Metropolitan Size
  6. Geographic Changes in Mode Share
  7. Metropolitan Density, Transit Share & Congestion
  8. “Fampools” & Social Trust
  9. New Americans & Carpooling
  10. Personal Income & Carpool Mode Share
  11. Non-Commute Carpooling
  12. International Commute Mode Shares
  13. Summary of Carpool Trends

Selective History of Ridesharing

Last modified on 2010-08-27 13:22:59 GMT.

It is interesting to note that there is not a substantial amount of information written on the history of ridesharing. Given the difficulty in measuring ridesharing, and distinguishing it from private automobile travel, this finding is not particularly surprising.

Jitney Craze: 1914-1918
The first historical incidence of ridesharing success was the tremendously popular yet short lived “Jitney Craze” beginning in 1914. In 1908, the Ford Motor Co. began offering the Model T, the first mass-produced automobile that was affordable to the average “successful” person. The vehicle’s popularity soared; in 1908 only 5,896 Model T’s were sold but by 1916 sales had soared to 377,036 nationwide (Hodges, 2006). With the increasing penetration of the relatively affordable automobile, streetcars faced their first real competition in the urban transport market. In the summer of 1914, the US economy fell into recession with the outbreak of WWI and some entrepreneurial vehicle owners in Los Angeles began to pickup streetcar passengers in exchange for a ‘jitney’ (the five cent streetcar fare). The jitney idea spread incredibly rapidly; by December 1914 (merely 6 months after the idea was believed to have been conceived) Los Angeles had issued 1,520 chauffeur licenses for jitney operation (Eckert & Hilton, 1972). In San Francisco, jitneys first appeared in 1914 and were first used to transport workers and attendees to the 1914-1915 Panama-Pacific International Exposition. By 1915, over 1,400 jitneys were operating in San Francisco (Cervero, 1997). With the first jitney’s beginning service in Portland, ME in March 1915, the jitney craze had spread from west to east in nine months (Eckert & Hilton, 1972).

Many of the original jitneys operated on well-known streetcar lines and effectively survived by siphoning off streetcar passengers. From the passenger’s perspective, the jitneys offered service improvements over the streetcar. Jitney’s often operated at speeds 1.5 to 2 times faster than streetcars (Eckert & Hilton, 1972) and could occasionally be convinced to deviate from main routes for drop-offs closer to passenger destinations. For passengers, the ability to choose between two service offerings for the same price was also an attractive service feature. While the reliability of jitney service was sometimes questionable (many only ran during peak periods, few ran during bad weather), passengers had a second option in the form of the streetcar. Travel time savings, route flexibility and transport mode choice were the major value propositions for passengers.

Jitney use was not without tradeoffs. Jitney drivers were known to drive aggressively and accidents were frequent. With passengers standing on the back of vehicles and on the running boards, serious injuries did occur. The transport of female passengers raised concerns in some social circles (Hodges, 2006).

An underlying question that remains a topic of debate is whether jitneys constituted a form of ridesharing or unregulated taxi service. To properly answer this question, the impetus for offering rides should be considered. Given the downturn in the economy, stories of unemployed persons purchasing a vehicle and becoming a jitney operator have been cited in the literature (Eckert & Hilton, 1972, Hodges, 2006). In these cases, jitney service could best be characterized as unregulated taxi service, as drivers were operating the vehicle for the express purpose of providing service to others. In other cases, jitney service seemed to be a method of offsetting the costs of private vehicle ownership for trips that were already going to take place. In a survey in Houston, TX on February 2, 1915, of the 714 active jitneys that day, 442 (62%) made only one or two round-trips, suggesting they might be operating as a jitney during their commute to and from work (Hodges, 2006). In these cases, the primary purpose of the trip was likely commuting; providing service to others was secondary. Any additional revenue generated simply offset the cost of vehicle ownership. In these cases, the generation of revenue was not the main purpose for operating the vehicle, so it could be argued that the service was a form of ridesharing.

The downfall of the jitneys was nearly as rapid as their rise. By early 1915, concerns over safety and liability were being reported in the popular press (New York Times, 1915). Streetcar interests and local governments were eager to stop jitney operations to limit losses in revenue. Streetcar operators were losing paying customers to jitneys, and local governments often taxed streetcar operators a percentage of revenue that they earned, so they were losing tax income as well (Eckert & Hilton, 1972). Many local governments implemented license requirements for jitneys, but the regulation with the largest impact was the imposition of liability bonds. Before operating, jitney drivers were forced to post $1,000 to $10,000 in liability insurance. The licensing and liability regulations added annual costs of approximately $150 to $300, or 25-50% of annual earnings for full-time jitney operators. By July 1915, twenty-seven localities had implemented liability regulations (Eckert & Hilton, 1972). It was estimated that of the 62,000 jitneys in operation in 1915, only 39,000 were still in operation by January 1916 and fewer than 6,000 by October 1918 (Eckert & Hilton, 1972).

There are several important reflections to be drawn from the “Jitney Craze” period. First, the original impetus for picking up passengers appears to have been due to the downturn in the economy. For those that already owned a vehicle, the offering of a ride was presumably to offset operating and ownership costs. For those that began offering jitney services during the period, many had been unemployed and saw operating a jitney as an employment opportunity. In either case, personal finance issues appear to have been a factor. Second, liability and safety were two of the major concerns with jitney service. These same issues remain major concerns with ridesharing today. Third, jitney service provision did not appear to be driven in any major way by resource constraints or environmental benefits, it was largely due to gaps in service quality and economic factors.

World War II: 1941-1945
The second major period of rideshare participation, and the period most likely to be identified as the first instance of traditional carpooling, was during World War II (WWII). In a reversal from the jitney era, government encouraged ridesharing heavily during WWII as a method of conserving resources for the war effort. This period of rideshare promotion was exceptionally unique in that it entailed an extensive and cooperative effort between the federal government and American oil companies.

European involvement in WWII began in 1939 but US involvement did not get underway until late-1941 and early-1942. Nevertheless, the federal government had begun making preparations for war much earlier. In May 1941, President Roosevelt established the Office of the Petroleum Coordinator (OPC) (US PAW, 1946). OPC was created to coordinate and centralize all government activities related to petroleum use. The Office was unique in that it relied heavily on industry committees to make recommendations to government; government initiated very few regulations (US PAW, 1946). This structure was chosen specifically because it encouraged all oil industry participants to cooperate amongst themselves, and it was felt that a more cooperative relationship with industry would lead to a greater overall level of voluntary effort.

By July 1941, one of OPC’s industry committees organized the first known petroleum conservation effort in the US. The campaign was launched on the East Coast with a $250,000 advertisement budget funded entirely by industry asking the motoring public to use 30% less gasoline (US PAW, 1946). Recommended actions included lowering drive speeds, proper care of tires and the sharing of rides. By the industry’s own admission, the effort was not terribly successful. A lack of public appreciation of the need to conserve fuel was cited as the leading challenge to be overcome; “this first drive to emphasize to the American people the necessity of gasoline conservation served one important purpose: it showed industry itself the magnitude of the task, and the growing need for a long-range, sustained program of public education” (US PAW, 1946).

In November 1941, industry created their official council that would interact with the federal government. The Petroleum Industry War Council (PIWC, originally named the Petroleum Industry Council for National Defense), a group that consisted entirely of petroleum industry representatives, was the entity that would ultimately design and fund all petroleum conservation activities during WWII (US PAW, 1946). In an odd twist of irony, PIWC held their first committee meeting on Dec. 8, 1941, one day after the attack on Pearl Harbor and the exact day that President Roosevelt signed the declaration of war against Japan. By February 1942, PIWC had established their Subcommittee on Products Conservation (under the Marketing Committee) and had completed the design of their nationwide conservation program by month’s end (US PAW, 1946). After a year and a half of effort, the Subcommittee was discharged in September 1943 and replaced by the higher-level Products Conservation Committee, suggesting the growing importance of oil conservation. This structure remained until the end of WWII. The Products Conservation committee’s programs had three main goals; (a) to provide the public with facts so that everyone might better understand the need for rationing, (b) to obtain better compliance with rationing programs, and (c) to bring about greater conservation of gasoline through car sharing [carpooling] and other measures (US PAW, 1946).

The Products Conservation committee was made up largely of advertising specialists. The bulk of the rideshare initiative (and all conservation initiatives during the War) focused on catchy slogans, posters and newspaper advertisements. While the PIWC spent considerable time developing some of the most recognized posters during WWII, they themselves did not publish them. OPC worked collaboratively with various government agencies including the Office of War Transportation, the Office of Price Administration and the Office of War Administration to distribute the ads that they created (US PAW, 1946). All advertisements were released to the public through a government agency. It is once again worth noting that the petroleum industry volunteered their time and resources to this effort with little financial support from government. At the end of the War, it was estimated that the Products Conservation committees had expended $8M. in private funding to support conservation efforts alone (US PAW, 1946).


While there is plenty of evidence of rideshare promotion during WWII, no information could be found on how successful the initiatives actually were. Some statistics exist on the number of newspaper advertisements placed and the estimated readership reached; however, little appears to be known about the actual level of ridesharing that took place during WWII. As discussed in the introduction to this section, part of the reason may be the lack of a financial transaction when sharing rides. Total auto use and transit ridership can be reasonably measured using financial transaction data, but rideshare participation cannot.

As with the “Jitney Era”, there are some important reflections to be drawn from the rideshare experience during WWII. In contrast to the “Jitney Era”, the main force behind ridesharing during WWII was government-mandated resource constraints (gasoline and rubber) rather than a market response to an economic downturn or a gap in transit service quality. Further, marketers during WWII understood that by appealing to patriotism they could encourage behavior change. There appears to have been a sincere belief that those remaining in the U.S. should make an effort to support their countrymen overseas by reducing their consumption and by making a behavioral change. Third, and perhaps most importantly, was the fact that the promotion of ridesharing during WWII was a large cooperative effort between the U.S. federal government and private industry. It is clear that interests of national importance took precedence over corporate interests during this time period, however, in the absence of a national emergency or compelling long-term threat, it is not likely that this sort of public-private relationship between government and the petroleum industry could be recreated today.

The 1970’s Energy Crises
The third period of interest and participation in ridesharing was during the energy crises of the 1970’s. Interest in ridesharing picked up substantially with the Arab Oil Embargo in the fall of 1973. Throughout the fall and early winter of 1973, President Nixon’s administration realized that action would need to be taken to reduce petroleum consumption. In January 1974, Nixon signed the Emergency Highway Energy Conservation Act, which mandated maximum speed limits of 55 MPH on public highways (Woolley & Peters, [2]). The Act was also the first instance where the US federal government began providing funding for rideshare initiatives. For the first time, states were allowed to spend their highway funds on rideshare demonstration projects (Woolley & Peters, [2]). The 1978 Surface Transportation Assistance Act would eventually make funding for rideshare initiatives permanent (US EPA, 1998).

It was during this initial energy crisis that states began experimenting with High Occupancy Vehicle (HOV) lanes. The Shirley Memorial Highway in Northern Virginia had opened as a dedicated busway in the center of the highway in 1969 becoming the first highway in the nation to provide dedicated infrastructure for High Occupancy Vehicles (Kozel, 2002). Beginning in December of 1973, vehicles with four or more passengers were also allowed to use the busway. HOV lane construction proceeded slowly through the 1970’s, with interest increasing in the mid-1980’s. While the number of required occupants was eventually decreased to three, the Shirley Highway remains one of the most heavily used HOV corridors in the nation today (Kozel, 2002).

The early 1970’s marked another first for ridesharing; it was the first time that it was recommended as a tool to mitigate air quality problems (Horowitz, 1976). The 1970 Clean Air Act Amendments established the National Ambient Air Quality Standards and gave the Environmental Protection Agency (EPA) substantial authority to regulate air quality attainment (US EPA, 2008). After initially rejecting the State of California’s ‘transportation control plan’ to meet Clean Air Act requirements in the Los Angeles basin in mid-1972, the EPA issued its own draft plan in late-1972 (Bland, 1976). The initial plan was met with substantial backlash, particularly a provision that would reduce gasoline consumption during the high-smog summer months by an incredible 86% through an aggressive gasoline rationing system. After public consultations, a much more moderate final control plan was issued in 1973. One of the main provisions of the final plan was a two-phase conversion of 184 miles of freeway and arterial roadway lanes to bus/carpool lanes and the development of a regional computerized carpool matching system (Bland, 1976). Phase One was to be completed by May 1974 and Phase Two by May 1976. By the 1976 Phase Two deadline, not a single lane-mile of roadway had been converted for high occupancy vehicle traffic (the El Monte Busway was opened in 1973 and allowed HOV 3+ in 1976; however, it was designed and operating as a high occupancy vehicle facility prior to EPA’s final control plan and therefore did not count as a conversion) (Bland, 1976). In fact, the next HOV project in LA County would not be constructed until 1985 and by 1993 there was still only 58 miles of HOV lanes countywide (LA MTA, 2009).

The post-1973/74 Oil Embargo period was a time of great interest in ridesharing. With the funding of rideshare demonstration projects in 1974, academic study of ridesharing and of the results of the rideshare demonstration projects began in earnest. The post-73/74 period also saw the creation of the nation’s first metropolitan rideshare agencies (US EPA, 1998). At the outset, these organizations relied largely on marketing campaigns encouraging ridesharing, largely disseminated through roadside signs and public service messages. As research into ridesharing progressed, the importance of employer-based initiatives became better understood and many agencies began to work more closely with large employers (US EPA, 1998).

By the late-1970’s, President Carter proposed multiple initiatives to further encourage ridesharing. In 1979, he appointed the National Task Force on Ridesharing to “expand ridesharing programs through direct encouragement and assistance, and create a continuing dialogue among all parties involved in managing ridesharing programs and/or incentive programs” (Downs, 1980, Woolley & Peters, [1]). His administration also understood the negative effect of parking subsidies on rideshare participation. In 1979, his administration tried to amend the National Energy Conservation Act (NECA) to eliminate subsidized parking for federal employees. The bill faced strong opposition from federal employees and was never passed (S. 930, 1979). In 1980, a bill was even introduced which sought to create a National Office of Ridesharing. As with the NECA amendment, this bill was never passed into law (HR. 6469, 1980).

The energy crises of the 1970’s marked a number of ‘firsts’ for ridesharing. It was the first time that the federal government formally funded rideshare initiatives, it was the first time that ridesharing was prescribed as an air quality mitigation strategy and it was the beginning of formal academic research into rideshare motivations and the potential to reduce petroleum consumption. As with previous periods though, national rideshare statistics were just starting to be gathered, making it difficult to determine how influential the energy shortages and government-sponsored programs had been on participation. Much like ridesharing in the WWII-era, the federal government’s involvement was largely a response to a resource shortage, in this case exclusively petroleum.

While many held strong hopes for ridesharing at the beginning of the 1980’s, low oil prices and strong economic growth throughout the decade and into the 1990’s dashed those hopes.

Defining Ridesharing

Last modified on 2010-08-27 13:13:40 GMT.

One of the major challenges in establishing a consistent mental image of ridesharing is the lack of a widespread, common definition of the phenomenon. Many existing definitions attempt to define ridesharing on the basis of passenger trip purpose, frequency of trip, commonality of origin & destination, and profit motive. A common point of confusion in defining shared vehicle transportation is how to distinguish between ridesharing, public transit and taxi services.

The distinction between ridesharing and public transit is not always clear. An argument could be made that the major differences between the two are the capacity of the vehicle, and the participant that determines whether a trip is undertaken (driver or passenger). The vehicle capacity distinction should be obvious; transit frequently (but not always) operates larger vehicles such as trains and buses whereas ridesharing often occurs in smaller vehicles such as cars and passenger vans. While transit’s primary purpose is to transport multiple passengers to a destination, ridesharing differs in that it generally occurs when the driver is planning on undertaking a trip and seeks out a passenger who is willing to share the ride. In other words, transit trips operate with an implied understanding that service is based on passenger demand and if sufficient communal demand does not exist (such as on weekends) the driver will not undertake the trip (service will be reduced). Ridesharing, on the other hand, is better described as a trip that the driver intends on taking whether or not they can find an appropriate passenger to share the ride with.

The “vehicle capacity” and “trip determined by driver or passenger” characteristics alone are not entirely satisfying. If one considers taxi trips, they take place in small vehicles where the passenger dictates the purpose of the trip. Essentially, taxi trips are neither a form of transit or ridesharing according to the distinction laid out in the previous paragraph. A further refinement is the absence of a profit-seeking motive on the part of the driver in rideshare arrangements. Whereas taxi drivers are profit seeking in their carriage of passengers, rideshare drivers in most cases seek only to share the costs of transport. In fact, in some jurisdictions such as Colorado, government legislation mandates that financial transactions only reflect the sharing of trip costs.

The three characteristics of ridesharing (smaller vehicle capacities, the trip is determined by the driver’s needs and the lack of profit motive) allows for the categorization of most forms of shared vehicle transportation. The figure below presents a categorization of shared vehicle transportation where driver/passenger trip determination is on the horizontal axis, profit motive is on the vertical axis and the size of the circles represents the relative capacity of the vehicles used.

Based on the shared transport characteristics described above, we have proposed a definition of ridesharing as follows:
“the transportation of two or more individuals in a motor vehicle with a capacity not exceeding 15 passengers, when such transportation is incidental to the principal purpose of the driver, which is to reach a destination, and when such transportation does not seek to transport persons for profit.”
This definition incorporates the three characteristics described earlier in this chapter. Further, it addresses the issue of “unsustainable ridesharing”, whereby an initial rideshare journey results in an SOV return journey because the trip is undertaken to meet the needs of the passenger. “Unsustainable ridesharing” is not uncommon in school trips, where parents will drive their child to school but will return home alone. Since the definition states that the driver’s principal purpose determines the trip being undertaken, multi-occupant trips catering to the passenger’s trip purpose should not count as ridesharing according to a strict interpretation of this definition.

This definition does create some important measurement limitations. The inclusion of driver trip purpose in the definition makes the identification of rideshare trips much more difficult, and certainly more onerous than simply counting vehicles with at least two occupants. Yet, the inclusion of driver trip purpose is a very important addition to the definition of ridesharing, particularly from a policy standpoint. Ridesharing is often described as a sustainable alternative to traveling alone and is encouraged by different levels of government. Clearly the sustainability of this mode rests on the ability to combine two unique trips that would have otherwise occurred separately. When travel demand leads to the creation of a new vehicle-based trip, part of which is an SOV trip, it undermines the message that ridesharing is a sustainable mode. Ideally, the measurement of rideshare participation should differentiate between multi-occupant trips undertaken based on driver vs. passenger trip purpose, and only assign credit to those where the driver’s trip purpose dictated travel. To operationalize this definition, more precise travel diaries and surveys would need to be administered specifically asking participants what the purpose of their trip was, and seeking more detailed information on trip-chaining tendencies.

Below are a variety of statistics that are meant to paint a picture of the current state of ridesharing. The majority of the data was retrieved from the 2000 US Census, the 2001 National Household Travel Survey (NHTS), and the 2005-2007 American Community Survey, the most recent comprehensive national travel data sets that are available. Note that all three sources refer to ridesharing as carpooling; the terms will be used interchangeably on this website.

Historical Trend – Journey to Work

Last modified on 2010-08-27 12:10:54 GMT.

Ridesharing as a mode of travel to work was relatively popular in 1970 and 1980, accounting for approximately 20% of work trips. The 1980’s were difficult for ridesharing; by 1990, nationwide rideshare participation had decreased by 3.6 Million commuters and mode share dropped to approximately 13%. Ridesharing reversed somewhat in the 1990’s and by 2000 had added back 250,000 participants. This increase however did not keep pace with the overall growth in commuters resulting in a decrease in mode share to just over 12% of trips to work in 2000. The downward percentage trend continued in the early 2000’s but appears to have reversed course by 2006, likely due to increasing petroleum prices.

Price of Gasoline & Disposable Personal Income

Last modified on 2010-08-27 12:53:10 GMT.

The relationship between high gasoline prices, disposable personal income and changes in aggregate, nationwide rideshare participation from 2002 through 2008 is interesting. As expected, changes in the cost of gasoline and rideshare participation tend to move in the same direction. The relationship between gasoline prices and rideshare participation has a Pearson’s R (a measure or correlation between two variables) value of 0.65, suggesting a reasonably strong correlation, but highlighting that there are other factors influencing rideshare participation. In some respects, the price of gasoline may not really be the underlying cause for rideshare behavior changes; rather, it seems likely that tighter household budgets would be more indicative. If one compares the year-over-year change in rideshare participation and per capita disposable personal income (in real terms), one can see that the two move in opposite directions, as one would expect; as disposable income decreases, rideshare participation increases. The Pearson R value is -0.62 suggesting a similarly strong, inverse correlation as was observed between real gasoline prices and rideshare participation. This is an important observation as it may suggest that non-transportation related strains on household budgets (such as an economic downturn, or lower wage growth) are as likely to influence rideshare participation as gasoline prices are. If this hypothesis holds, one should expect to see continued high levels of ridesharing in 2009/2010 due to continued economic weakness, even with the substantial decreases in retail gasoline prices.

Mode Share by Metropolitan Size

Last modified on 2009-02-24 13:42:42 GMT.

In stark contrast to transit use, carpool mode share is very consistent across metropolitan areas of different sizes. Not surprisingly, non-metropolitan region (rural) journey to work trips had a higher carpool share, as these commuters likely have longer commutes, have fewer transport options and are more likely to achieve cost savings from carpooling. Overall, this finding seems somewhat counterintuitive; one would have expected non-metro regions and large metro regions to have higher shares, as commuters in these areas are more likely to benefit from ridesharing through reduced fuel consumption and travel time savings. Although only speculation on our part, the lower-than-expected carpool mode share in large metro regions may be partly due to the larger transit service offerings.

Geographic Changes in Mode Share

Last modified on 2009-02-24 15:02:38 GMT.

The geographic changes in ridesharing from 1990 to 2000 are quite pronounced. During the decade, only four metro regions of over 1 Million people had increases in carpool mode share and they were predominantly west of the Mississippi (Seattle, Phoenix & Dallas. Atlanta was the fourth). The map below paints an interesting story. While carpool mode share has been decreasing nationwide, the largest decreases have been in the eastern US. At the metropolitan level, the results are even more pronounced; of the top 10 metro regions with the highest carpool mode shares in 2000, eight are located in the US Southwest (CA, NV, AZ, NM & TX). The 10 metro regions with the lowest carpool mode shares were all in the Northeast (NY, MA, CT) and the Upper Midwest (OH, MI, PA). It is also interesting to note that the two metro regions that are frequently cited as examples of carpool success stories (San Francisco & Washington, DC) rank 16th and 21st respectively in terms of carpool mode share. Ironically, three cities often criticized for their reliance on the private automobile (Phoenix, Los Angeles & Las Vegas) have the highest proportion of carpool commuters in the US.
[Note that this analysis was limited to metro areas with 500,000 commuters or more]

Metropolitan Density, Transit Share & Congestion

Last modified on 2009-04-16 07:34:13 GMT.

As was seen in the previous section, geographic differences within the US appear to have some effect on rideshare participation. One potential reason for this is differences in metropolitan population densities. Cities in the Northeast developed much sooner than cities in the South and West, and were not originally designed to accommodate private automobiles. The chart below suggests that density may have an influence on carpool mode choice; as population densities increase, carpool mode share falls. What this chart does not provide any information on is whether density itself leads to decreasing carpool mode share, or whether higher densities improve the viability of other modes of transportation (such as transit) leading to a mode shift away from carpool.

Indeed, if one considers both carpool and transit mode shares and compares it to metropolitan density, the picture becomes a bit clearer. At higher densities, transit is the dominant mode choice (low carpool / transit ratios) while at lower densities carpool is the dominant mode choice (high carpool / transit ratios). Intuitively this makes some sense if one believes that higher population densities are a prerequisite for viable transit service.

[Note: The density calculations in the two previous graphs were done at the MSA level. MSA's in the US are determined by county boundaries rather than any sort of density gradient. As such, MSA's that include large counties with a primarily rural population (many in US Southwest, for example) will have lower densities than their actual urbanized area.]

One of the implications of the previous charts is that carpool and transit appear to compete for mode share. As we’ll see later on in the International section, this appears to be at least somewhat true at an aggregate level. However, its important to realize that this relationship is anything but certain. The chart below plots metro region carpool and transit shares against one and other. If the relationship between carpool and transit were strong, we would expect to see a pattern of dots sloping from top-left to bottom-right. In actuality, the data does not show any particular relationship between carpool and transit mode share at the metropolitan level.

Shifting from the relationship between transit and ridesharing to congestion and ridesharing, we see that at the aggregate level there is a consistent, positive trend. Those metro areas with higher levels of congestion generally have higher carpool mode shares. The trend is more pronounced for large metro areas than it is for smaller ones.

For metro areas with HOV facilities, the presumption is that as metro level congestion increases, commuters form carpools to take advantage of less congested HOV lanes. In these cases, there is an obvious travel-time savings benefit for the driver and probably for the passenger. In instances where HOV lanes are not present, the impetus for carpool formation is less clear. The argument has been made that some commuters choose to ride as a passenger in heavily congested situations to avoid the stress of driving. From an economic standpoint, there is less of an impetus when HOV lanes are not present. This may also explain the lack of trend in the small and medium metro areas; although no analysis was performed, one can assume the majority of freeway HOV facilities are found in the larger US metro areas.

“Fampools” and Social Trust

Last modified on 2010-08-27 11:58:07 GMT.

In 2000, 77% of nationwide carpools involved two individuals (the driver and one passenger). Of these two-person carpools, it is believed that 25-80% are “fampools”, carpools comprised of family members, depending largely on trip type.

Attanucci (1974), in a survey of commuter carpooling in Boston, places the percentage between 25-45%, with co-workers representing another 50-70%. Kendall (1975), in a survey of Boston commuters found that 35% of carpools were intra-household. However, recent sources place the percentage of commuting involving family members much higher. Pisarski (2006) suggests the percentage is closer to 80% but provides no supporting evidence. Li (2007) in a survey of Texas carpoolers found that approximately 65% were family members, with co-workers representing another 30%. Morency (2007) using survey data from Montreal, Canada found that 82% of carpoolers were family members with another 9% representing co-workers.

Note that the Attanucci and Kendall surveys, while several decades earlier, were surveys of commuter carpoolers specifically, and found a much lower share of household members and a much higher share of co-workers. The Li and Morency surveys focused on all trip types and found much higher intra-household participation.
Intuitively, it is expected that commuting trips would involve smaller percentages of family members than trips for other purposes. Carpool statistics from MIT tend to support this; approximately 59 of 234 employees (25%) with registered carpool parking permits are family members. Note that this percentage is an understatement of true “fampools” at MIT, because employees that are dropped off at campus by a family member cannot be identified through parking permit registration. However, the statistics certainly suggest that “fampooling” for commuting trips is much lower than the 65-80% quoted elsewhere.

Beyond considerations of trip type, it is clear that the percentage of carpooling occurring between unknown participants is rare; Attanucci (1974) found 3%, Li (2007) found 3-5% and Morency (2007) found approximately 9%.

New Americans & Carpooling

Last modified on 2009-02-24 16:49:35 GMT.

In 2000, foreign-born commuters accounted for slightly less than 14% of the working population, yet they made up nearly 20% of the share of 2-person carpools and over 40% of the share of carpools with 5 or more people.

However, it would be incorrect to characterize carpooling as simply a domestic vs. foreign issue. The evidence suggests that recent immigrants are much more likely to carpool than those foreign-born residents that have lived in the US for some time.

Ignoring length of time in the US for the time being, the 2000 Census data also shows that the incidence of carpooling is much higher among Hispanic Americans than it is for any other ethnic group. White, non-Hispanic Americans’ carpool share is only 10% for commute trips while the share for African Americans is 16% and the share for Hispanic Americans is 22%. This finding begins to explain the high rideshare mode splits in the metro areas of the US Southwest where Hispanic populations are more highly concentrated.

The high share of carpool activity attributed to recent immigrants and Hispanics leaves an interesting question unanswered. Is the initial high participation rate in carpools due to an inability to purchase a private vehicle (for financial reasons or otherwise), cultural factors related to shared vehicle transport, some combination of the two or other factors entirely?

Personal Income and Carpool Mode Share

Last modified on 2009-02-24 17:07:22 GMT.

There is a believe that carpooling has an inverse relationship with personal income; as income increases, families purchase additional vehicles, single occupant vehicle trips increase and carpool participation decreases. Indeed, national level journey to work data shown in the chart below confirms this inverse relationship between personal income and carpool share; carpooling as a share of mode split decreases as income increases. This is in contrast to transit mode share, which first decreases with increasing income, but increases in the higher personal income brackets.

The metro-level relationship between personal income and carpool share is basically the same as the national level trend. In almost all cases, metro region carpooling decreases steadily with increasing income. Dallas-Fort Worth, TX is a good example of this. Carpool mode share for those making less than $25K is nearly 20%, yet decreases to just over 5% for those making more than $75K. Washington, DC is interesting in that it is one of only two cities in which carpool mode share increases (albeit marginally) in the highest income bracket. Seattle, WA is perhaps the most interesting metro are from a carpool mode share standpoint. Its carpool share remains remarkably consistent over all income brackets, only showing a noticeable decrease for those in the highest income bracket. It is likely that Washington state’s Commute Trip Reduction legislation for large employers explain some of Seattle’s higher than average carpool mode share in higher income brackets.

When the metropolitan level data is analyzed over time, the income trend is not significantly different; the change in inflation-adjusted household income at the metropolitan level has a discernible, but varied impact on carpool mode share. One can certainly see that decreases in carpooling have commonly occurred when household incomes have risen (top-left quadrant) however there are instances when decreases in carpooling have been associated with decreases in income (bottom-left) and more recently, increases in carpooling have been observed when income has increased (top-right).

While it appears that there is a substantial change in mode share in the 1980’s as compared to the 1990’s, one should remember that the 1980 Census would have been taken during a period of high gas prices resulting from the 1970’s Energy Crises. With the sharp decrease in gasoline prices in the 1980’s, it is not surprising that the carpool share showed a large decrease across many metropolitan areas.

Overall, the data suggests that increasing levels of income are associated with a decrease in carpool mode share, both at the national and metropolitan levels.

Non-Commute Carpooling

Last modified on 2010-08-27 11:45:09 GMT.

All of the statistics quoted thus far have been based on Home based Work (HBW) trips, or commute trips in other words. Looking at carpool and vehicle occupancy rates for all trips (including HBW) shows a very different story. According to the 2001 NHTS, multi-occupant vehicle trips (i.e. carpool trips) account for 48.9% of trips as compared to 12.6% of work-only trips. This is an interesting fact to consider when developing a rideshare strategy. While lower occupancy HBW trips and work-related trips only account for 16% of total trips in the US, they cause a disproportionately high percentage of the congestion experienced nationwide. Shifts from SOV to HOV for weekday commute trips are likely to have larger positive impacts on congestion than similar shifts in non-commute trips. This would seem to suggest that ridesharing initiatives should focus on commute-based trips.

Occupancy for different trip purposes is only one aspect of capacity utilization; one must also consider differing trip lengths by trip purpose. The following chart shows total annual seat miles of unused capacity per vehicle for different trip types. So while only 16% of total trips nationwide are commute trips, they represent 33% of total seat-miles of unused vehicle capacity (and 46% of peak period seat-miles of unused vehicle capacity). This reinforces the previous belief that ridesharing initiatives should focus on commute-based trips. However, one should also consider the differing values of time that users place on different trip types. Commute trips are likely to have a higher value of time for most drivers, thereby making any inconvenience or delay in their journey incredibly onerous. If ridesharing is to be improved during the commute, convenience for drivers and passengers, and minimal delays are essential.

International Commute Mode Shares

Last modified on 2009-02-24 17:30:20 GMT.

It may surprise some to learn that ridesharing as a percentage of commute mode share in the US is relatively high compared to other developed countries. The table below shows the mode split for commute trips for the US, Canada, the UK and Australia. The US has a substantially higher level of ridesharing than any of the other three nations. It is interesting to note however, that all three other nations have substantially higher public transit mode shares than the US does. If the two modes (carpool & public transit) are considered together, the combined share is quite similar across all countries. This may suggest that the two modes compete with one and other, as suggested at several points on the website. Further, this assertion is supported by the fact that the strongest rideshare markets in the US have relatively low transit shares, and vice versa.

Summary of Carpool Trends

Last modified on 2010-08-27 13:43:25 GMT.

This summary is certainly not comprehensive, it is a simple tally of the trends described above.

1. Ridesharing has a strong cultural and social aspect to it. The largest group of ridesharers are family members, co-workers and neighbors. Certain groups, namely Hispanic-Americans, share a large proportion of rides in the US.

2. Changes in Disposable Personal Income, whether due to higher gasoline prices, lower overall earnings or an economic downturn, appear to have a significant effect on aggregate rideshare participation. Rideshare surveys tend to support this finding, with participants often listing cost savings as one of the three main reasons for sharing rides.

3. At an aggregate level, rideshare participation appears to be higher in low density metropolitan areas, but the reasons for this are not entirely clear. Intuitively, ridesharing requires at least some density of riders with similar origins and destinations in order to work. However, higher densities are often accompanied by larger and more interconnected transit systems that may compete for rideshare participants. For lower density metropolitan areas, ridesharing may function as a type of small vehicle transit.

4. Ridesharing and transit are likely complements and substitutes. The international analysis of transit share and rideshare participation suggest (at a very high level) a potential substitution between the two modes, however the metropolitan analysis did not show any significant trade-off between transit share and ridesharing in the US context. There is evidence from San Francisco that people who choose to rideshare previously rode transit, suggesting that the two modes are substitutes. On the opposite side, evidence from Seattle, San Francisco & Washington DC suggest that when passengers cannot find an appropriate ride-match, they rely on transit services to reach their desired destination, suggesting that the two modes complement one another.