How to Drive Slower, But Travel Faster!
By Michael Brown, PE, AICP
Overview: How do you serve high volumes of traffic, yet still have a desirable, walkable, livable “Place?” Engineers have creative new strategies to reduce congestion that they call “Innovative Intersections.” Some of their new strategies are irredeemably auto-oriented, but three designs have real potential for Place-Making.
This first article introduces the concept of “Drive Slower, Travel Faster” with the math that planners and architects can use when working with engineers to justify lower speeds. Subsequent articles discuss attributes of key Place-Making designs: Quadrant Intersections, Thru-Turn Intersections, and Town Center Intersections.
Can the tortoise win the race? Watch the video before reading this article.
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Engineers build dangerously fast “stroads” (access-oriented “street”/auto-mobility “road” hybrid) trying to balance land access needs with high traffic volumes. Communities want to reinvigorate these older, decaying, high-speed arterials as “Complete Street, Walkable Boulevards” with room for bikes, pedestrians, transit, and landscaping. But such stroads often have extreme congestion. If engineers and other stakeholders believe these plans will make a bad situation worse, then Place-making desires can end up hollowed out or just die with no political path forward.
But now there are win-win strategies that can can help turn stroads into walkable streets, while at the same time accommodating the same or even greater numbers of vehicles with less congestion.
What’s the magic? It’s what we call “Place-Making Innovative Intersection” designs. If any of these are good fits for your situation, there’s a good chance you can shake hands with engineers rather than wrestle for the upper-hand.
What if you could reduce speed limits, narrow lanes, or even eliminate a lane, and still give drivers faster average speeds? New opportunities for Complete Streets and Economic Development would emerge.
But such wishful thinking is impossible, isn’t it? If not, surely the world would have discovered the magic by now? Decades ago engineers invented many intersection designs that can handle more cars with the same number of lanes. Sounds like an auto-oriented solution, right? And that’s how engineers have tended to approach it. But some designs have features with amazing Place-Making potential, and need not be auto-oriented if approached with “Place” in mind.
Engineers often install left-turn arrows at intersections for safety. But this creates a lot of inefficiency and a different set of safety hazards, so the common-thread of all innovative intersections is to eliminate these left-turn phases. There are more than a dozen designs in the family. Some are irredeemably auto-oriented, such as the Continuous Flow Intersection. But the family also includes potential Place-Makers such as Quadrant Intersections, Town Center Intersections, and Thru-Turn Intersections for high-volume settings, and Roundabouts for mid-volume settings.
But before reviewing their place-making potential, the next section shows some of the math behind how more auto-capacity per lane can potentially be used to reduce the auto-oriented nature of streets. It’s the math that can help you negotiate with engineers.
A typical suburban intersection with left-turn arrows will get congested at fairly low volumes. You might wait more than 5-minutes, inching forward watching the signal turn red/green several times before you even reach the front of the line. Innovative intersection designs can often get the average wait at these signals down to a minute or two.
So which is faster? Racing at 50 mph on a “stroad” only to wait 3-5 minutes at signals? Or a 35 mph boulevard where you wait just a minute or so at intersections? Engineers may want both the intersection improvements and the historic stroad speed limit. But if you reach out to them they may concede that today’s situation would still be faster for drivers even if speed limits are reduced. So use intersection delay reduction as a bargaining chip for reducing the speed limit, narrowing lanes to 10 feet, and other traffic calming features. That’s what it means to drive slower, but travel faster. It’s win-win for autos, bike/ped, and economic development.
Innovative Intersections boost lane efficiency. A typical suburban arterial might carry about 700 vehicles per hour per lane. But if revamped to include innovative intersections, the boost might jump to 1,100 vehicles or more.
So how is accommodating more cars with the same lanes transit friendly? The increased capacity doesn’t always need to be for cars. In this chart, suppose you have an arterial with 3-lanes each direction where each lane can carry 700 vehicles per hour per lane (blue), or 2,100 total.
So suppose you’ve asked to convert the 3rd lane to transit, but engineers say it can’t be done without harming traffic flow. Now you can show them how innovative intersections might help the remaining two lanes each carry 1,100. The result would be 2,200 for autos (about the same as before), plus transit, for a much higher overall “people-capacity.”
If it works out you can’t quite give up a lane, you might be able to get “parts of lanes.” Engineers usually want 12-ft lanes. That’s their standard for stroads signed at 45-mph and higher. But if efficient intersections reduce delay, then you might reduce speeds to 35 without hindering average speeds. And 35 mph is where a new 10-ft standard kicks in. That’s the best-practice width for walkable boulevards supported by the Institute of Transportation Engineers, Congress for New Urbanism, and National Association of City Transportation Officials (ITE, CNU, and NACTO).
Try out StreetPlan.net, a free Complete Street cross-section design tool that uses “red/yellow/green” to guide you through context-based best-practices for speeds, lane widths, and other street attributes.
Turning Suburban Intersections into “Place-Makers”
Ideally, urban intersections would be small with modest volumes of traffic. That is possible in environments with high connectivity and good local grids. But suburbs tend to lack connectivity, and thus the few through-streets become overwhelmed with traffic, degrading adjacent land-uses quickly. Many believe Complete Street retrofits would allow land uses to rebound, but it is daunting to create walkable, livable boulevards and at the same time accommodate high volumes of traffic that has no where else to go.
One-way couplets (Town Center Intersections), Michigan-lefts (Thru-Turns), and left-turn diversion routes (Quadrants) were first invented by traffic engineers as a way to manage more vehicles. Couplets are very common, but often thought of as inherently auto-oriented when they need not be. With narrower lanes and other traffic calming techniques, couplets can prove to be better Place-Makers than two-way streets. Michigan-lefts are now spreading fast beyond Michigan, but they always seem to end up auto-oriented when they may not need to be. Quadrants are just starting to grab engineer’s attention, but few are aware of their place-making potential and how to tap it. Please learn about them so you can speak up!
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