Houston’s freeways were built for Downtown-bound traffic. The freeway and tollway network is almost perfectly radial, reflecting the fact that City-led freeway planning predated the state highway department’s involvement. The initial construction occurred between 1948 and 1972, when Downtown was the only game in town. Hines’s Galleria opened in 1970; Schnitzer’s Greenway Plaza in 1973.
Every freeway approaching Downtown loses about half its capacity to a series of braided ramps connecting to one-way streets. The remaining through capacity then mixes it up in a series of three interchanges. It is a robust, resilient design, which avoids having a single point of failure. Among US cities, only Los Angeles and Kansas City possess similar layouts; the same topology was extensively used in Eastern Bloc metro systems such as Prague, Kiev, and lines 1, 2, & 3 of Moscow and St. Petersburg.
A more common arrangement is to concentrate downtown traffic on a single central spine, as is done in Seattle, Denver, Atlanta, and Miami. The proposed reconstruction of IH-45 and the Downtown Houston freeway ring deletes the Pierce Elevated in favor of a much-expanded 59 and 10, essentially replicating Atlanta.
Downtown, and the Pierce Elevated in particular, are fairly congested even outside of rush hours. This happens for three reasons.
First, the assumption that half of all traffic is bound for Downtown no longer holds. As Houston has sprawled into an ever more multi-polar urban agglomeration, the Downtown freeway ring has shifted to a more regional role. Downtown has also been the primary benefactor of most transit improvements, including three light rail lines and a billion dollars’ worth of barrier-separated HOV and bus facilities. Over 30% of Downtown workers commute on transit, while the regional average is less than 4%. As a result, the reverse commute on IH 10 and US 59 is now worse than the traditional commute.
Second, the Downtown Ring was designed before modern standards for merges were developed. The original design relied extensively on abrupt center merges, and most of these have been removed. This has made the ring safer, but less able to process crosstown traffic.
Third, most of Houston’s growth in the last 50 years has occurred west of IH 45 and SH 288. The East and Eastex Freeways, which form the northern and eastern segments of the loop, are relatively uncongested relative to the region as a whole. This tends to concentrate a plurality of crosstown traffic onto the Pierce Elevated.
To visualize this, here’s Google’s “typical” traffic for 5:30pm on Friday, culled from the averages of smartphone users running Google maps.
All of the macro trends are visible here. The inbound Katy and Southwest Freeways are stop-and-go as traffic queues up to process through the Downtown Ring. The inbound Gulf and South Freeways are largely at free-flow, but these two have a smaller queue where traffic waits to get onto the Downtown Ring.
Once this traffic has merged, things flow smoothly. The northbound Pierce Elevated flows well past the merge from 45/59/288, while the southbound Pierce flows well once the traffic from Houston Avenue and Allen Parkway has gotten on board. How bad is this bottleneck today? One way to answer this question is to compare the lane count approaching downtown with the lane count passing through. By neglecting the myriad ramps to “Downtown Destinations”, we can have a rough measure of the downtown ring’s ability to process crosstown traffic. At the south end, this looks like so:
Three lanes of 59 enter after the Spur leaves the mainline, while four lanes of 288 merge in. The inbound Gulf Freeway loses two lanes to the 59/288 system, but future construction will relocate the 59 connection to the existing high-level collector-distributor, freeing up the ability for three lanes to continue. We can subtract an additional lane for traffic from 59 and 288 bound for the outbound Gulf Freeway, leaving 9 total entering lanes and 7 receiving lanes.
At the north end of Downtown, the situation is similar:
Four lanes of the North Freeway mix with four lanes of the Katy Freeway. Where 10 and 45 traffic merge into 10, three lanes continue. 45 picks up more, but loses several lanes to downtown, leaving three heading into the dual-sided merge with Houston and Allen.
The current proposed schematic for the reconstruction of IH 45 envisions a redo of the Downtown Ring (large PDF). This design promises to relieve congestion by adding lanes around Downtown. The problem is, it adds more lanes coming into Downtown than it does going around it. The upcoming 288 Managed Lanes project will add still more radial capacity. As a result, the percentage of lanes available for use by crosstown traffic is actually reduced.
Here’s the north end again:
By connecting the existing HOV skyway into a new crosstown managed lane, and by converting the existing exit-only to 45 north into an optional lane for 45 north or south, the current schematic brings the number of approaching lanes on IH 10 to six. Adding managed lanes on 45 likewise brings that number to six. However, both 45 managed lanes are connected to the existing Milam Street ramp, with a one-lane connection to the mainline provided in the southbound direction only. Still more lanes are diverted into the Downtown Connector, a mirror of the 59 Spur which preserves the direct ramp to City Hall.
By the time all of these ramps have left the building, only six free lanes plus the managed lanes remain. This reduces crosstown capacity to less than 60%. The south side isn’t much better:
The current schematic eliminates the inbound 59 bottleneck at the Spur, by continuing five lanes through Midtown. However, the same bottlenecks are kept downstream; there’s a short weave with 288 traffic before two lanes go onto 45 north, merging with two lanes from the Gulf Freeway and then tapering down to three – virtually the same configuration as today.
Any transport improvement will bring with it some “induced demand.” The improvements represented by the 45 managed lanes, the 288 managed lanes, and the removal of the 59/Spur bottleneck will lead to more cars entering the Downtown Ring. If the past 20 years are a guide, an increasing number of those trips will be crosstown and reverse commuters. By substantially increasing capacity into Downtown without a commensurate increase in capacity through and around Downtown, we are creating tomorrow’s bottleneck.