The AASHTO Guide has some parts that mix motor vehicle design parameters with those for bicycles -- for example in their suggestions on trail design. This can cause some odd results if Guide users aren't careful. The most glaring error in the AASHTO Guide involves sight distance, a problem that goes back to the 1981 version. Clearly someone's been asleep at the wheel since it hasn't been corrected in well over a decade!
[If you have a copy of the 1991 Guide, you can use the page numbers to follow along with this example. If you have the '81 Guide, you'll be able to find the same charts and quotes on other pages.]
Step 1: Assume a standard design speed for a flat trail of 20mph (p.25). This is AASHTO's recommended minimum speed.
Step 2: Using the minimum radius chart on p.26, pick the proper curve radius. It's 95 feet.
Step 3: Note the caution in the last paragraph of p.27: "Bicyclists frequently ride abreast of each other on bicycle paths and, on narrow bicycle paths, bicyclists have a tendency to ride near the middle of the path. For these reasons, and because of the serious consequences of a head on bicycle accident, lateral clearances on horizontal curves should be calculated based on the sum of the stopping sight distances for bicyclists traveling in opposite directions around the curve." Seems logical, since we don't want people running head-on into each other.
Step 4: Using the chart on p.28, determine the minimum stopping distance for the grade of 0 percent and a 20mph design speed: approximately 125 feet. Based on the caution mentioned in Step 3, double this to 250 feet.
Step 5: Using the graph on p.30, determine the necessary lateral clearance to an obstruction on the inside of the curve, given a radius of 95 feet and sight distance of 250 feet. Now you probably notice the first indication that something's wrong:
If you extend the lines so that they do meet and eyeball their intersection with the vertical scale, you get a lateral clearance to obstructions of about 71 feet (M in diagram below). This can also be derived using the formula found on the same page.
Step 6: To figure how much clearance you'll need from the edge of pavement for a standard trail width of 10 feet, first determine the distance from a bicyclist's location centered in the inside lane of the curve and the inside pavement edge of curve:
Step 7: Subtract this figure from your lateral clearance to obstructions of 71 feet and -- surprise -- you're going to have to clear everything back from the inside of the curve to the tune of about 68.5 feet from the edge of pavement. Holy clearcut, Batman! AASHTO Guide, the logger's best friend!
Clearly (no pun intended), we can't seriously use the AASHTO Guide's recommendations when providing sight distance on curves. At the same time, blind curves on trails can lead to head-on crashes, as AASHTO suggests. What to do? Until AASHTO revisits their manual and replaces motor vehicle-derived formulas, here are some thoughts...
First, it's a good idea to separate the two directions of travel in turns. For this reason, providing a solid yellow centerline should be standard on all curves. While I like the idea of striping the entire length of a trail, curves, at least, should be done. Next, it's important to provide at least the minimum shoulder (2 feet) and clearance to obstructions (3 feet). In addition, curve widening (up to 4 feet) can also help, as can warning signs if nothing else can be done.
In terms of sight distance, itself, get as much as you can without using a "scorched earth policy." And if your design calls for a long curve around a stand of trees, for example, consider a modified alignment that reduces the severity of the curve, as shown below. While this isn't always possible, it's surprising how often designers provide sharp and blind curves when it's not absolutely necessary.
Guide for the Development of Bicycle Facilities; 1991; by AASHTO Task Force on Geometric Design.
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