Wednesday, December 23, 2015

PV109 Access Permit: The Three Speeds

The Albertini subdivision at 109 Pleasant Valley Road is, to put it politely, problematic.

There are three kinds of speed involved when considering an access permit, and in any situation where road design is at issue: design speed, operating speed and posted speed. One of the goals in engineering a road is to ensure that these three measurements are roughly equal. The process is supposed to go something like this: Figure out how fast you want traffic to flow, design the roadway to that target design speed, study the actual operating speed (85th %ile), set the posted speed to match the operating speed.When these three speeds converge, the road is relatively safe and comfortable.

Especially since reconstruction of the section of Pleasant Valley Road at Mountain Road, the posted speed along Pleasant Valley Road is not consistent with the operating speed of the road. It is arbitrarily too low.

The Vermont Agency of Transportation's Traffic Impact Study Guidelines succinctly explain the problem of arbitrarily low posted speed limits:
Much like installing traffic signals, reducing speed limits is often seen as a panacea for traffic ills. Speed limits must be set in accordance to the MUTCD. The MUTCD, in section 2B.13, requires than an engineering study be completed in accordance with established engineering principles. It states that the speed limit should be set within 5 mph of the 85th percentile speed of free-flowing traffic. Other factors which may be considered along with the 85th percentile speed are road characteristics (geometry), pace speed, roadside development and environment, parking practices and pedestrian activity and reported crash experience.
Since the 85th percentile speed is quite often higher than the existing speed limit, it is difficult to justify a lower speed limit. Arbitrarily setting a lower speed limit is not an effective method to reduce driver speeds – drivers select their operating speed by “feel”, with little regard to the posted speed limit unless there is active enforcement. Assuming that the majority of drivers are prudent and safe drivers, the 85th percentile speed is deemed a reasonable speed limit unless there are factors which strongly suggest that drivers are making poor decisions at that speed.  
This idea that design speed, operating speed and posted speed should be equal is a fundamental notion in traffic engineering. A lot of design decisions and traffic analysis rely on -- and flow from --this idea. When these three speeds are not in agreement, bad things happen.

That is why it is so disappointing that the consultancy attempting to develop a subdivision at 109 Pleasant Valley Road (PV109) did not immediately point out the discrepancy between posted and operating speed.

Here's how easy it is to discover:
1) Point your browser to the CCRPC database
2) Under Traffic Data, click on Map
3) Click on Underhill in the map of Chittenden County
4) Click on the Underhill Center region of the town, it will look like this:

Each of the dots represents a traffic count location. Click on the one near Pleasant Valley Road at Mountain Road, UHIL16. Then click on "Speed Profile" (more details available by clicking "Download Count"). You will see that even in 2005, before the straightening, flattening and widening project, the operating speed (85th%ile) is well above the current posted speed of 30mph.

At this point, knowing that the posted speed is 30mph, alarm bells should be ringing in the mind of a traffic engineer. But that's not what happened.

Trudell Consulting Engineers (TCE) has repeatedly understated the traffic and safety issues with the proposal to construct an intersection for the proposed Applewood Lane. All the while many, many people have expressed concern about the proposal to construct an intersection on the inside of the blind corner south of Mountain Road. Even now, TCE has proposed a close -- too close -- offset-T intersection.  At earlier stages of the process, they attempted to substitute posted speed for design speed and argue that a 4-way intersection with Mountain Road is not feasible. Now, with the 4-way design complete, they have concocted a "possible class III wetland" and lean on it as a rationalization to continue to push for the offset-T.

Sadly, this lapse of engineering judgement and ethics is not an isolated incident.

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