I'm updating our transportation element, with a general policy section first, which is supposed to inform other decisions, namely our Major Street and Collector Plan. The street plan was last updated in the mid 90's, and from my understanding, was based on a typical 4-step travel demand model. As a result, the plan calls for everything from 10-lane freeways to 6 and 8 lane surface arterials, regardless of existing or planned adjacent land use.
As a transportation planner trying to balance transportation needs with land use/community concerns, 6 and 8 lane arterial surface streets seem excessive to me. I've read work by Walter Kulash, Ian Lockwood, Rick Hall and other engineers arguing that urban arterials shouldn't exceed 4 travel lanes, in consideration of ROW costs, crossing distance for pedestrians and safety of drivers changing lanes. This was echoed by my former MPO director in Greenville, SC, which has a handful of 7-lane arterials (6 travel lanes, plus center turn lane), which can be maddening to drive on in consideration of high speeds and erratic lane-changing.
Overall, it seems like communities have to put a cap on lane capacity at some point if they're going to preserve and enhance land use and other objectives. Is it unrealistic to cap existing arterials at their 4 travel lanes? The main challenge to this is that Nashville's road system is set up from radial pikes, with limited and often opposed network connectivity between them.
Still, 6 and 8 travel-lane roads conjure up highway conditions, not anything approaching an urban environment that supports walkability or varied land-use. I'm curious for thoughts.