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Our solution for Chandler Mill: 12 drivers shave 5 mph to save $5M

Updated: Feb 21, 2023

Why sacrifice 400 trees to shorten a trip by 21s for the 15% who speed?


Most people would agree that Chandler Mill is one of the prettiest stretches of road in the township. So it is no surprise that you will find people walking, running, and cycling on it every day, sharing the road with cars. The challenge when cars, pedestrians, and cyclists want to share Chandler Mill Road? Helping the minority of drivers who speed to just slow down!

While many will doubt that this can be done, we already have proof that traffic calming can slow drivers on sections of Chandler Mill down by 6-10 mph. We have a solution detailed elsewhere that builds on this proof, and that will certainly save 400 trees and hundreds of feet of sensitive streamside habitat, and that could take $4-5M off of the final bill. The solution rests on ensuring that the 1 out of about every 7 drivers who speed - e.g., those who drive 35 or more in a 25 mph zone - drop their speed to at least 30 mph. And because the traffic on Chandler Mill is so light, and probably consists mostly of area residents, we might only need a dozen neighbors to drive 30 instead of 35 mph to save us all up to $5M. Let’s break this down.


Engineers err on the side of caution when planning for safety, so instead of designing roads for the average driver (the 50th percentile), they over-engineer them, by designing for those who speed (the 85th percentile). When engineers refer to the prevailing speed, this is the 85th percentile. So while the average speed was about 30 mph south of Oriole Dr. (after the speed limit was re-posted to 25 mph), the 85%ile was about 35 mph. This is the design speed of the roadway used in all kinds of engineering calculations.


This small difference has a big impact. There is a general consensus that roadways cannot be comfortably shared with cyclists and pedestrians of all ages and abilities unless most drivers (eg., at least 6 out of 7) keep their speeds below 30 mph. Higher speeds are especially problematic wherever there are sharp curves - if drivers cannot see far enough ahead, they might have to pull out or brake suddenly for a pedestrian or cyclist as they round a corner. Even though there is no history of accidents on Chandler Mill, this experience might understandably make these most vulnerable users never want to use the road again. As we will discuss in a separate post, the problem is not the actual risk of a accident (because this is extraordinarily low) but the irrational fear of one.

Over-engineering on curves creates another problem. Since curves are designed with the prevailing speed in mind, they are tilted in ways that might help keep cars sliding off of the road. This is called superelevation (think of a NASCAR racetrack, like the picture to the right). If this tilt or cross-slope is greater than 2 degrees, it becomes impossible for wheelchair users to stay straight and not drift sideways. And on some of Chandler Mill's curves, this superelevation is 3 times what is actually needed.


A critical feature of the traffic on Chandler Mill is that it is remarkably light - an average of 158 vehicles per day were counted on the road in the November 2021 traffic study, or less than 80 round trips. Let's assume that most of these drivers either live on Chandler Mill or one of its side roads or are regular visitors, and complete one to two round trips per day. With only about 1 in 7 drivers clocking 35 mph or greater, than means that Chandler Mill's speeding problems may be due to only 6 to 12 neighbors or their friends per day.

So our solution detailed elsewhere is deceptively simple: Drop prevailing speeds to 30 mph, and to 25mph on the 3-4 sharpest curves to make sure that drivers have plenty of time to slow down for pedestrians. Dropping the speed to 25 mph in the sharpest curves also helps us limit super-elevation to 2 degrees, making the road comfortable even for wheelchair users. We expect that most of this can be accomplished by removing the center line and using speed cushions (see above). Drivers can comfortably cross these at 25 mph, and emergency vehicles (which have the wheelbase to comfortably straddle the 6' wide cushions) do not have to slow down at all. We can begin to pilot the speed cushions and explore the benefits almost immediately.


Only those really speed would be impacted by these changes! The average driver would see no difference - they would still be able to comfortably complete the 1.2 mile journey from Round Hill to Hillendale at an average speed of 25 to 30 mph. For those used to driving at 35 mph, reducing their speed by 5 mph lengthens their trip by about 21s. So do we really need to spend millions of dollars, cut down hundreds of trees, destroy hundreds of feet of streambank, and leave residents worried that Kennett Township will seize some of their land for a sidepath, just to shave 42 seconds off of the round trip for 15% of residents living in these neighborhoods?


There is another reason to try this solution out: if it works here, it can work elsewhere in Kennett! We have identified other roads in Kennett that, if shared, could unlock exercise and active transportation routes benefitting hundreds of households at almost no cost. Our proposed Fair Mills Loop, for example, could connect at least 200 households - and potentially hundreds more - to the 160 acre Spar Hill preserve.


We understand that the solution needed here probably goes beyond engineering. Changing public perceptions about the safety of shared roadways and changing driver’s behavior is more complicated. But the greatest obstacle right now is probably political - namely, Kennett’s fervent wish to avoid yet another fiscal fiasco by changing course. Changing the design now is costly, because of the embarrassment of having spent hundreds of thousands on a design that cannot be completed. But we believe it would be even worse to spend 5 times that amount on a plan that will not even deliver substantive benefits to cyclists. Over the coming months, we will learn whether Supervisors are able to set their embarrassment aside for the public good.

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