Bham, I think your impressions of those systems may be somewhat outdated. DART [[Dallas) now has the longest light-rail network in the US, plus 121 fixed bus routes and a downtown streetcar. The first light rail segment opened in 1996 and the latest extension in 2014. By comparison I count 81 total bus routes between DDOT and SMART. And can you imagine 93 miles of light rail in SE Michigan? That's enough for routes from downtown to Pontiac, Mt. Clemens, DTW, and Novi, with a little left over for an extension from downtown to Belle Isle.
Houston just did a major redesign of their entire bus network with the goal of increasing frequency of service in huge areas of the city. Ridership is already up despite changes to huge numbers of existing routes. Here's a link: Houston transit, reimagined. Houston has also built three light rail lines since 2003 and has a BRT line under construction. Transit seems to be a much higher priority in Houston than it has been in Detroit.
Phoenix just approved a $32 billion transportation sales tax increase in 2015 [[recall that the RTA proposal was for $4.7 billion) and has numerous extensions to its own light rail line under construction or planning. Again, seems like a significant difference compared to SE Michigan.
I agree with you that causation in any direction is weak or tricky to establish at best. High-capacity transit like rail supports dense development, and this can mean it gets built after the fact because other modes are too crowded or that it gets built in anticipation of dense development. Bus service probably tends to reflect the priority that regional leadership puts on public transit more generally, since it performs a wide range of functions including serving both core transportation routes and lifeline service in farther-flung areas. But this all just goes to the point I was trying to make earlier in the thread. It's simply a political decision to make transit a higher priority, not something hard coded by population growth, population density, city history or anything else.
Edit: One other interesting fact I read a few months ago. The population of Houston as it existed in 1960 has actually decreased, and indeed the neighborhoods south of downtown Houston rival Detroit's east side for abandonment. All of the "growth" came from absorbing outlying suburbs.
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