How Fast will Vehicles Travel on Your Street Design?
High speed is deadly in an urban environment. If hit at 20 mph, 9 of 10 pedestrians survive. But if hit at 40 mph, only 1 in 10 survive. So how fast are vehicles likely to go on your design? StreetPlan offers an excellent sketch-planning methodology for ballpark estimates.
Our method starts with the typical "Stroad" above, and adjusts factors up or down. Stroads usually have 12-ft lanes, two lanes each direction, a center turn lane, mostly empty shoulders, few trees, and auto-oriented adjacent land uses. On a stroad, if the speed limit is 40, most drivers will go up to 50. The fear of getting a ticket is what finally stops most drivers from going much more than 10 mph above the speed limit.
Estimating New Speeds: First depict your existing cross-section and posted speed limit, and StreetPlan will show what it thinks "Your Speed" is likely to be - the 85th percentile speed were you to take an average with a radar gun. Next, create an alternative cross-section. If you narrow the lanes, Your Speed goes down a bit. If you increase the number of lanes each direction, Your Speed goes up a bit. If you install a planted median, speed goes down. If you depict on-street parking (implying it will be heavily utilized), speed goes down. If you depict street trees or mixed-use buildings, (implying a lot of trees and a lot of buildings near the sidewalk), speed goes down. You can also check a box for speed bumps, raised intersections, and other traffic calming strategies that are not depictable in StreetPlan.
What Factors Influence Speed?
Detectable from Cross-Section
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Speed Limit (Fear of Ticket)
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Number of Lanes
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Width of Lanes
Detectable from Cross-Section
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Median Type (planted, empty, etc.)
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Shoulder size, parking, bulb-outs
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Proximity of trees and buildings
Checkbox Options
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Speed Bumps, Raised Crossings
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Pavers, Cobblestone
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Artistic Pavement, and more!
Are These Speed Predictions Reliable?
StreetPlan is a sketch-planning tool, so take everything "with a grain of salt." There are many reasons why your situation may vary measurably. However, Urban Innovators used a radar gun to determine the 85th percentile speed on many different kinds of roads and streets, and recorded the parameters that contributed to the observed speed. We also researched available literature on the topic. After that, we created an algorithm for estimating speeds. Then we constructed many cross-sections in StreetPlan, and adjusted parameters until "Your Speed" was coming pretty close to the measurements we observed in the field and in literature.
How Do You Use This Feature?
At the top of this dialog box is a checkbox where you can turn on or turn off the Your Speed overlay. If you turn it on, you will also see a "Three Dots" button. Click that and you can see all of the parameters that affect speed, along with how powerful they are at reducing the overall speed relative to the default Stroad. The more factors that make your design different than the datum (the Stroad shown above), the more that Your Speed will also be different.
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Note that StreetPlan uses a decay function for each additional factor. The most powerful single factor gets 100% credit for its stand-alone effect, but each factor thereafter has less and less of its stand-alone effect. In the end, all factors are multiplied together for a composite factor, which is then multiplied by your target speed limit. It is possible that Your Speed will end up lower than the target speed limit. If so, great! You've achieved compliance.
Yellow vs Red: If predicted speed is <= 5 mph above the limit, Your Speed will be yellow. If it is >= 6 mph above the limit, Your Speed will be red. If Your Speed is below the limit, it will still be yellow.
