Responses to Speed Hump Questions
RESPONSES TO SPEED HUMP QUESTIONS
by Joseph Cutro, PE, Traffic Engineering Consultant
Wednesday, December 1, 2010
Here are responses to Peter Gray's questions about speed humps in the context of the village-wide plan that is currently under consideration. Please regard this message/memo as a supplement to my original report dated September 10, 2010.
Q1: What are the pros and cons of separating the "core" group of humps on four streets from the group of secondary locations? Would there be any significant downside from considering both at the same time?
A1: According to my original recommendations, implementation of the "core" humps would be a prerequisite for installation of a hump at any secondary location. This speaks only to the issue of whether Section 5 will have speed humps in the first place. I see no other good reason to preclude the secondary locations from "doing their own thing". By the way, on the basis of input received at the November 9 meeting, I have modified my thinking a bit on having the core group as a village-wide prerequisite. I would now recommend that Section 5 unconditionally cooperate and participate in any speed hump projects for the secondary locations that might be initiated outside the village, e.g.; Leland Street by Montgomery County, East Thornapple by Martin's Additions.
I see no problem in allowing residents of the secondary locations to participate in the consensus process for the core group. Those residents can serve as a reasonable proxy for non-residents (Rollingwood?) whose travel paths might actually be affected by the humps. Otherwise, where do you draw the consensus participation line for non-residents? If there is concern that residents unaffected by the humps would have too much influence on the consensus result, the supermajority requirements for approval can be lowered from those required by the County. I have recommended a single village-wide approval supermajority of 60%.
On the other side of the coin, consensus participation for each of the secondary locations should be limited to residents living on the respective street in question, or its "traffic-shed". [e.g., Alden Court is within the traffic-shed of Glendale Road.] The rest of the Village would not participate in the consensus process itself, but other residents could always submit advisory comments outside of the actual "voting".
So in response to the second part of the question, I would not consider both groups at the same time. Other than determining if Section 5 will have speed humps at all, it would be patently unfair for the larger community to be able to overrule the will of the individual street's residents. All other area programs leave the consensus solely in the hands of the residents deemed to be directly affected.
Q2: Why would 35mph, rather than 25 or 30mph, be considered "egregious" speeding in a posted 20mph limit zone?
A2: Traffic engineers would define "egregious" speeding statistically, comprised of speeds ranging from as low as 5% (95th percentile) up to 15% (85th percentile) of the highest speed observations. Within that range, unfortunately, there is little agreement about how fast is too fast. In best practice, the observed 85th percentile speed should be the basis for the posted speed limit, which ideally should be set at the 5 mph increment below the measured 85th percentile speed. If the speed limit is set arbitrarily, which it often is, there may be little or no relationship between the posted speed limit and what is truly excessive speeding.
I'm guessing that your definition of "egregious" is based on VVRC's 2007 study of speeds and volumes on Section 5 streets. That study highlights a column listing the number of speed observations above 35 mph, which at 10 mph above the most-used (overwhelmingly so) posted speed limit in these parts, is a basic measuring stick in many area traffic calming and police enforcement programs. For Section 5's narrow streets, such a definition of "egregious" would appear to be too high. A speed of 35 mph represents only just over 1% - the 99th percentile - of speed observations for Section 5's "core" streets. From the VVRC study, we also know that the 85th percentile speed is right around 24 mph for all four streets. That would place the 95th percentile, a level of speed that almost everybody would regard as excessive, at about 30 mph.
By the way, whether by design or by luck, your 20 mph speed limit on these streets has been set properly. That value is the 5 mph increment below the measured 85th percentile speed, and is also about 10 mph below the "egregious" 95th percentile speed. If you wish, I can provide the Village with professional certification for this limit, as mandated in and by Maryland Vehicle Law.
Q3: Is the combination of a hump with a speed limit sign more effective in slowing traffic than a hump without a speed limit sign?
A3: I've never seen a study on this, but I feel certain that the answer is no. Prevailing traffic speeds are dictated primarily by the roadway's physical characteristics - width, curvature, presence of on-street parking, and added features like speed humps. With or without a speed hump, regulatory (black on white) speed limit signs only affect prevailing speeds to the extent of police presence to enforce them. Although not terribly effective, the signs remain necessary for enforcement to take place. Alternatively, the speed advisory plates (black on yellow) usually used with speed humps are not enforceable at all, and simply advise drivers of the appropriate speed at which to traverse the hump. The MUTCD states that an advisory speed plate should be placed beneath the usual BUMP sign at each hump, or, placed in advance of the first hump in a series. For U.S. standard 12' speed humps, an advisory speed value of 15 mph is appropriate.
Q4: What speed hump design is more effective in slowing speed, circular or flat-topped?
A4: By "circular", you probably mean what is actually termed "parabolic". The U.S. standard 12' hump usually takes a parabolic profile, although an alternative "sinusoidal" shape is also permitted (although difficult to construct and infrequently used). The parabolic model, appropriate for all Section 5's local streets, has a "median design speed" of about 18 mph. The U.S. standard flat-topped "speed table" is 22' in length. It's median design speed is about 23 mph, and is more appropriate for higher-level "collector" streets, a class in which Leland Street prospectively falls. With its lower design speed, the parabolic hump is the more effective at slowing traffic.
Q5: Are there any safety concerns with installing speed humps on streets which slope up or downhill?
A5: Yes. The ITE guidelines ("national standard") state that humps should not be placed on street grades exceeding 8%. The Village's steepest grades appears to be those on Woodbine Street, both east and west of Glendale. In my estimation, these grades appear to be in the range of 6%. I can perform a more formal check of these grades if you wish.
Q6: Is there any evidence that speed humps will generate an increase in noise or pollution when trucks, buses, vehicles with trailers, etc. cross over them?
A6: Studies from Colorado and California indicate that speed humps reduce overall traffic noise by about 4 dbA, due primarily to reduced operating speeds. These reductions can be locally (near the hump) offset by the noise of braking and ensuing acceleration, and also that emanating from the movement of loose objects within vehicles like pickup trucks and service vans.
Q7: What is the optimal speed limit for humps? Nearby communities' limits vary from 10mph (Martin's Additions) to none (Section 3).
A7: Presumably, you're referring to the advisory speed plates that should accompany the humps. As stated above, the "optimal" posted speed is 15 mph. The 10 mph advisory speed plates used in Martin's and other jurisdictions are frankly speculative attempts to fool motorists into driving at yet lower speeds. The plates are totally ineffective in that capacity, but are harmless from a liability standpoint. The 15 mph plate is the most truthful in informing the motorist of the condition of road ahead, and therefore should be preferred. Thanks for the tip about Section 3 (also a customer), and I will inform its manager about the potential liability of the missing supplemental speed advisory plates.
I'd also like to use this message as an opportunity to amend one of my original recommendations regarding the prospective Connecticut Avenue turn restrictions. From input received at the November 9 meeting, it became clear to me that there needs to be at least one additional "cordon" option on the table, one that specifically protects Leland Street in addition to Section 5's other east-west through streets. This "Plan 2" would place right turn restriction signs (effective during the selected PM peak period) at 4 more intersections, besides the seven intersections already included in what could now be called "Plan 1". The additional locations would be:
- northbound Connecticut Avenue at Leland Street, with an exception for transit buses,
- northbound Connecticut Avenue at Blackthorn Street,
- eastbound East-West Highway at Glendale Road,
- eastbound East-West Highway at Curtis Street.
Under Plan 1 (7 intersections), eastbound Leland Street would be the cordon's northern edge and effective bypass for both local and through traffic. Under Plan 2 (11 intersections), the edge/bypass function would be shifted to southbound Brookeville Road. Plan 2 obviously suggests the need for further coordination with Montgomery County government, which has jurisdiction over the additional streets to be protected.