As part of the IH-345 tear-out study, I've been wondering something: of the 160,000 vehicles per day on 345, exactly how many vehicles are actually coming to downtown? This is critical in its relevance. Otherwise, having the corridor there, merely adds congestion, pollution, and a burden upon nearby real estate that otherwise should be more valuable given its proximity to downtown, DART, etc.
From speaking with John Norquist, current president of the CNU and former Mayor of Milwaukee, he told me that when the Westside Highway fell in Manhattan and it was replaced by a surface boulevard (the gap in this diagram illustrating a highway nearly wrapping the entire perimeter of Manhattan island), that 53% of the traffic disappeared as it was using the highway as an express route from New Jersey to New Jersey. It was fast and convenient, particularly for people not paying taxes nor living in a New York City borough.
Similarly, when Central Expressway in San Francisco suffered structural damage due to the 1989 Loma Prieto earthquake in 1989 and it was replaced with Octavia Boulevard, 25% of total traffic load disappeared. 50% re-routed to different highways. It wasn't actually headed to the vicinity, but merely cutting through. Traffic congestion improved despite traffic planners dire warnings.
You would think this kind of information would be critical to evaluating the best solution to what to do with our own on-going structural failure of IH-345. We have to think about this stretch of road within the context of a broader network, its impact on real estate development patterns near and far, tax base, long-term infrastructural costs and burdens, and particularly potential benefits beyond simply preventing disaster, ie keeping the thing from falling down with the weight of a few hundred tractor trailers backed up on it at 5 pm on a Friday, suspended 100 feet in the air.
So, without going to the extent of dropping counting cables at every on- and off-ramp in downtown along 345 (though this ought to be done), I took what data I could find at various locations where it is available. In the map below, I indicated the traffic count and location of the available data points, ignoring anything to old to be relevant, like traffic counts from the '80's.
The apricot color shows on-ramps, off-ramps are in Burnt Sienna for you Crayola 64 enthusiasts. Ignore the faded white corridors for now, but keep in mind that those are tremendously over-scaled roads that operate well-under capacity.
There are three on-/off-ramp access points to/fro 345: Ross, Live Oak/Bryan/Pearl, and Elm/Main/Commerce on the downtown side. Poor Deep Ellum gets none and likes it. There's a fourth, being Cesar Chavez running beneath 345, but I'll get to that later.
So, realizing this data is incomplete, I interpolated to roughly determine total on-/off-ramp traffic counts. For example, Ross Avenue has "ons" and "offs" in both directions. A total of four, but the data here is extremely limited with only two counts, made even worse in that the numbers are the exact same. So for this I took the one number and multiplied it by four. Unfortunately, it's the best I've got.
Counts are available at all of Live Oak, Bryan, and Pearl. As for Elm/Main/Commerce, as you can see above, there are two on's and two off's that fork. Unfortunately here, we only have counts for one side of each fork. For this, I doubled each and summed (4172x2 + 2832x2).
Using these numbers, I created the infographic below:
Here you can see the totals exiting or entering 345 at the three highway to surface grid junctions. Adding all of them up, you get a total of 40,297 or 1/4th of the traffic count for 345. Cesar Chavez on-ramp count is 11,900 and off-ramp of 11,129 for a total of 23,000. However, we can exclude this as this portion of Cesar Chavez is never really using 345 above. It is merely accessing or exiting 75 to the north. The total of 23,000 would be there if it were simply a surface boulevard and there were no elevated highway above.
Obviously, there are caveats. However, this is an attempt to improve the quality and understanding of the big data at a fine-grained scale, and what it actually means to downtown. And what happens with downtown effects Dallas as a whole. For the most part, 345 is a means of moving from one highway to another, large destination to large destination, not the fine-grained network supportive of the increased density and tax base the city of Dallas rightly wants and needs.
By this rough count only 25% of the traffic of 345 is coming to or leaving downtown Dallas. Given the two-way nature of trips. Even with contingency built in, we'll never get anywhere near 50% or more, ie some number worth having the highway there. The low exit/entrance counts and Cesar Chavez show that the urban street grid has plenty of capacity and necessary route choice to access the existing regional highway and interstate network without 345.
Density, via demand and desirability, is built through a combination of local plus global interconnectivity. 345 is a barrier to local connectivity and doesn't improve the global/regional connectivity for existing and potential new residents of the area. With DART and the street grid leading to the other freeways in the area, the regional connectivity is still there. However, through removing the freeway and restitching the grid we can create new neighborhood opportunities for all that expected and estimated new growth.