Flexible Flow Lanes

If commuting makes us unhappy, then commuting in congestion, doubly so. In Houston, we’re trying many different ways to ease congestion including HOV lanes and the new HOT (a.k.a. managed) lanes on the Katy Freeway. But highway congestion is just one piece of the puzzle. Congestion occurs on the collector roads as well.

The common form of congestion occurs during rush hour when the majority of drivers are driving in the same direction. In general, congestion occurs inbound in the morning and outbound in the evening. This is common sense stuff, right? So my question is, why do we only use half of the available lanes during the periods of peak congestion?

If we have, for example, a six lane collector road, it is a waste to only use three lanes in the morning while the opposing 3 lanes carry minimal traffic. Why couldn’t we just have built a 4 lane road where 3 lanes carry the directional load during rush hour and at other times, the flow is split 2-2?

This kind of flexible-flow roadway isn’t applicable everywhere, but it can accommodate more traffic with fewer lanes in a few situations. I’ve seen flexible-flow lanes used when adding lanes is not an option due to geographical or property boundaries.

Take for example, Greentree Road four miles southwest of Pittsburgh, PA. Specifically this section north of Carnahan Rd. and south of Manilla Ave. This stretch of road serves as a collector to I-376 (Penn Lincoln Pkwy). I traveled this unique roadway every time my family visited my grandmother in the authentic, walkable borough of Dormont, PA. But I didn’t realize its special features until years later, when I drove it myself.

The distinguishing characteristics of this section of Greentree Road are the dashed, double-yellow line lane dividers and the overhead signals at intersections and non-intersections alike. Per U.S. road surface marking conventions, a solid double-yellow line indicates a division between traffic of opposing directions in which passing is not allowed. However, a dashed double-yellow line is not standard, it alerts the driver that something is different. The fact that both lines are dashed indicates that the two lines may or may not indicate a division between opposing traffic. Or, put another way, dashed, double-yellow lines sometimes indicate opposing traffic.

Greentree Rd, a flexible flow street in Pittsburgh, PA

Lit signals at intersections and along straight segments then clear up the meaning of the lane markings as well as answer the question “What lane(s) am I allowed to be in?” The signals providing obvious symbols either allowing or prohibiting traffic in the direction the viewer is facing. Traffic always stays to the right, it is just a question of how many lanes are available at any given time. The example traffic signal in the images below use a red ‘X’ to indicate that traffic is prohibited in that lane, while a green arrow indicates that traffic is allowed.

Prohibits traffic in this direction.	Allows traffic in this direction.

In this case study, a flexible-flow lane saved a quiet neighborhood road from being turned into a four-lane thoroughfare while still allowing two lanes of rush-hour traffic and one lane contra-flow. This technique of increasing the traffic load by time-sharing a lane is not suitable in all locations, but should be considered in some cases as an alternative to widening a road.

If you didn’t know already (as I didn’t until Andrew informed me after he reviewed this article), Houston has a road with a flexible flow lane.  Hop on W Alabama St at S Shephard Dr and head east.  It is a three lane road with the center lane becoming directional for rush hours and a turn lane during non-rush hours.  The flexible-flow lane continues 1.7 miles east the 527 spur where the road returns to a typical 4-lane plus a turn.

Alabama St was once mostly residential, but the combination of traffic and the conversion of many homes to businesses turned Alabama St into a more significant throughway.  Former lawns are now street-adjacent parking and established trees shade the sidewalks and road.  Alabama St. is a neighborhood friendly business corridor providing a calmer alternative to Richmond Ave and Westheimer Rd which surround it. The following image is typical of W Alabama St:

W Alabama St, Houston, TX

To conclude, a flexible-flow lane is a proven way to move more cars in both directions without widening a street beyond what is appropriate for the setting.  On-road markings and above-road signals manage the flow appropriately, but drivers unaccustomed to this traffic pattern will have to pay extra attention as head-to-head traffic is a possibility.  Despite the possibility of driver error, I expect to see more flexible-flow lanes proliferate as established urban areas deal with automotive congestion.