Double Yellow Line Danger on I-170 (Updated)

In a previous post, we saw how the lane shift at Tamm avenue caused drivers to wander out of their lanes. At least they were all going in the same direction. Recent changes on I-170 near Highway 40 have resulted in two way traffic separated by just a double yellow line. Let’s see how drivers handle this situation:

In the few minutes I watched traffic at I-170 and Galleria Parkway, nearly 50% of cars in the outside lane touched or crossed the dashed white line. Fewer crossed the double yellow line, but it only takes one car to do it at the wrong time before you have a serious head-on collision (like this crossover accident on I-64 in Illinois this week). It seems that southbound drivers are approaching this area too quickly to negotiate the lane shift to the right without wandering out of their lanes.

Median Crossover GuidelinesI looked at the federal guidelines to see what they specify for such a situation. Although these guidelines are always subject to interpretation, the guidance I found would recommend use of channelizing devices instead of just the double yellow line. The figure to the right shows the guidance for median crossover on a freeway. It specifies as a standard “Channelizing devices or temporary traffic barriers shall be used to separate opposing vehicular traffic.”

Another section of the federal guidelines addresses a similar situation to I-170, but just for a total of two lanes:

When two-lane, two-way traffic control must be maintained on one roadway of a normally divided highway, opposing vehicular traffic shall be separated with either temporary traffic barriers (concrete safety-shape or approved alternate) or with channelizing devices throughout the length of the two-way operation. The use of markings and complementary signing, by themselves, shall not be used.

Tubular channelizing deviceBased on what I’ve seen at I-170, I would recommend avoiding this section of roadway until some sort of channelizing device is put into place to separate opposing traffic. The device shown at the right could be put in place with little impact on spacing. If you are concerned about this section of roadway, share a comment here, but, most importantly, let the I-64 construction team know by leaving them a comment on their web site. I will be.

Update 07/06/07:

The team at was once again prompt and professional in their response to my concerns about this section of I-170. Here’s what they had to say:

The area you describe is not a typical interstate construction zone. The freeway currently ends and begins at Eager Rd. In the current construction configuration, it operates as a typical 4-lane arterial roadway, with the double yellow line separating the north- and south-bound traffic. Traffic in this area is posted with a workzone speed limit of 45 mph. Vehicles in this section are approaching a stop or just leaving from a stop, so the interstate standards are not relevant. A channelizing device is not required for that type of roadway condition.

I agree that northbound traffic is leaving a stop and is more cautious going through this construction zone. However, it’s the southbound I-170 drivers that think they’re on a interstate when they enter this construction zone. That area around Galleria Parkway sees high speed southbound drivers, and I do think that some channelizing device would reduce the risk of a head-on collision.

Prediction: WB Highway 40 will not jam at I-170

As mentioned in the introductory post for this blog, I will have regular posts that predict how traffic will flow after the Highway 40 construction is complete. Not that I have any credentials to do such predictions, but, honestly, it’s fun, and if I’m wrong, well people will have forgotten about it by December 31, 2009 anyway.

I thought I’d start with an easy prediction: Westbound 40 will flow more smoothly at I-170 and not jam as regularly as it did pre-construction. Let’s at least hope so! If we’re spending $535 million to improve the highway, one might expect to it flow a little more smoothly.

In order to back up this prediction with some analysis, let’s look at what was causing the regular westbound 40 jams pre-construction. The key is to look at where the jam clears up, not where traffic begins to slow down. For westbound 40 during the evening rush, the slowdown might start as far back as the Science Center, but it would always open briefly right between Hanley and I-170 before another (sometimes brief) slowdown just west of Brentwood. So, we need to focus on the Hanely/I-170/Brentwood area.

The map above shows the key chokepoints on Highway 40 pre-construction. Traffic from Hanley road onto westbound 40 was forced to merge quickly and criss-cross with traffic that was destined for northbound I-170. All of this lane changing in a short time slows down traffic, and the backup just propagated further back as the evening rush volume increased. Once all the cars had completed their lane changes, traffic would open back up again just past these two chokepoints, until traffic from southbound I-170 and Brentwood tried to squeeze into the three lanes of already-near-capacity traffic on westbound 40. This caused the final chokepoint in the daily commuter’s trek along westbound 40.

The design for the new Highway 40 eliminates these chokepoints, which will lead to a much smoother flow of traffic through the I-170 interchange. The video below, courtesy of Gateway Constructors, shows how westbound 40 will look after the construction.

Video from courtesy Gateway Constructors (Problems viewing? View directly and fast-forward to the one minute mark.)

To view the designs up close, take a look at the Brentwood/Hanley/I-170 plans from If you zoom in, you will notice two key points:

  • No direct merges from Hanley to westbound 40 — this eliminates the first two chokepoints, since Hanley traffic won’t merge until after traffic bound for I-170 has already exited. (A future prediction will look at what this means for Hanley and Brentwood)
  • Additional lanes to support merging westbound traffic — Plans had always talked about an extra lane west of I-170, but in fact there will be two extra lanes between Brentwood and McKnight, giving plenty of space to absorb traffic joining westbound 40 from I-170, Brentwood, and Hanley.

Interestingly, construction crews have already eliminated the Hanley access points to westbound 40. If it weren’t for the large lane shift at I-170, I suspect you’d already see fewer backups on westbound 40, due to the elimination of those short merges and criss-crosses.

Overall, this is a very positive new design that should make for more pleasant evening commutes at this complicated interchange.

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