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.