Mysteries of Howard intersection unraveled
A western Howard County reader, who preferred to remain unidentified, had a detailed question abbout why the inttersection of Marriottsville Road and Route 99 is configured as it is. The State Highway Administration came back with an equally wonky answer.
I'll pass along both, wiith the caveat that it's probably far too much information for anyone who doesn't live out that way or travel there frequently. That is -- unless you want to get a flavor of the sheer complexity of the decision-making that goes into the design of any intersection.
As you can see, whatever action is taken at an intersection produces a rreaction that is not always predictable -- even for professional engineers. The reader wrote:
There is an intersection less than a mile from my home that my family and I use daily. It is located at Md. Rt. 99 and Marriottsville Rd. When we moved here in 1977 it was a 4-way stop sign. Then, over the years, it graduated from a yellow blinker (due to citizens tired of the constant accidents petitioning) to, finally, a stoplight.
There have been scads of accidents there over the years. Traffic has gotten really bad there now, particularly at rush hours. People travelling to and from Carroll County use it as a thoroughfare alternative to Rt. 32, which is getting ever more congested, as both roads run north to south.
The problem now is that there is a green arrow to turn left from northbound Marriottsville Rd. onto Rt. 99 (west). The arrow is operational at all times EXCEPT when it's needed the most!!! Which is from about 4 P.M. to about 6:30 P.M., PRIME rush hour time. The traffic really backs up, sometimes to the Rt. 70 bridge. Plus, the visibility turning left without the arrow is tricky. Southbound Marriottsville Rd.
at the intersection has 2 major lanes, and cars stacking up in the left lane wanting to turn onto Rt. 99 east block the view. Particularly if SUVs are there, they make it impossible to see what's behind them.
YET..............this is what ticks me off, the green arrow works every other time, including, say, 2 in the morning and all night.
About 4 years ago, the SHA had a community meeting to discuss widening Rt. 70 from Rt. 29 westward, and I attended. I cornered one of the SHA guys to ask why the green arrow light wasn't on at rush hours.
He said it was deliberately timed to not be on during rush hour, and I remember some lame-brained excuse why.
This is beyond belief. It's a no-brainer to have the green arrow on at rush hour to expediate travel during the busiest time of day. The line of cars is unreal.
I'm asking you, please, if you can help rectify the situation. Can you ask and find out why SHA doesn't have it on, and if they could reset the timer? Thank you.
So SHA spokesman David Buck talked with the local engineers and came back with this reply:
The reporter is inquiring as to why we do not operate the signal as a split-phase during during the p.m. peak hours. With lighter traffic volume on S/B Marriottsville Rd in the p.m., this is first we have heard there is an issue for N/B drivers to make a left-turn. While we would not operate Marriottsville Rd as a split in the p.m. peak for the capacity reasons described above, there may be an opportunity to provide a brief leading left-turn arrow on N/B Marriottsville Rd, similar to what is done in the morning.
Our engineers will gather some additional data and have a recommendation in about a month.
On a related matter, the Howard County Department of Public Works is working on a project to widen Marriottsville Rd from US 40 to north of MD 99 in conjunction with several developments in the area. The County's plan would provide a 5 lane section along Marriottsville Rd extending just north of MD 99. Once this project is constructed, there will be separate left-turn lanes on N/B and S/B Marriottsville Rd at MD 99 and the signal will have E/P left-turn phasing at all times.
You may want to contact Howard County about their timeline.
Again, I apologize for this being so lengthy but it does indicate: we have made adjustments over the years in response to customer concerns; this is an extremely busy intersection and any tweaks will have effects on other movements; and we are going to investigate your reader's concern and see if there is something that can be done to help.
And there you have it: Everything you could want to know about this intersection and more.
Comments
To clarify Mr. Buck's comments: "split phasing" is when two opposing legs of the intersection (Marriottsville Road, in this case) are given the right of way independently/sequentially, rather than concurrently. Hence the term "split".
When the left-most signal head is a four-section head, with a circular green and a green arrow that illuminate concurrently, you know it's a split phase approach. Since it is "split" from the opposing through movement, left turns can always be made without yielding to oncoming traffic, hence the lack of a yellow arrow for an independent left turn phase that you see in the commonplace five-section signal heads. For instance, most of the intersections on US 40 through Ellicott City (Rogers, Saint John's, Chatham, Bethany/Centennial) are split phase for the side street.
Marriottsville & MD 99 has an "issue", in that the lane configuration for the Marriottsville approaches is a shared through and left lane, adjacent to a right only lane. There is no dedicated lane for left turners to wait for a gap in opposing traffic, so through traffic stacks up behind them, unless they dart into the right turn lane temporarily. And you can see the potential for conflict there. Therefore, you typically only see shared through/left lanes at split phase locations.
Split phasing is a very inefficient mode of operation, but it does have its place. As Mr. Buck alluded to, the insane amount of northbound traffic in the PM rush mandates that split phasing be temporily suspended. I've seen Marriottsville Road back up in the PM past Warwick Way to nearly I-70; imagine how much worse it would be if northbound Marriottsville had to wait 15-20 more seconds every cycle for the piddly amount of southbound traffic. I'm not saying that this makes life any easier for the poor guy on southbound Marriottsville waiting to turn left onto MD 99 EB, but...traffic engineering is a balancing act, what can I say.
In my humble opinion, there is a relatively low-cost option to deal with this intersection in the short term, prior to Marriottsville Road being reconstructed: Marriottsville Road could be restriped to have a dedicated left turn lane and a shared through/right lane on both approaches (which is what MD 99 has now). This would allow for non-split phase operation at all times, and left turners could be given their own phase. I don't see the need for the dedicated right turn lanes as they exist today.
Posted by: Traffic Engineer | May 11, 2010 8:05 AM