Wednesday, August 29, 2007

The bicycle's scoop on gravel

Looks like the concern was a false alarm, thank goodness. My apologies for the rant. It turns out that whether you use D-1 or borrow on an actual road will make a big difference in how long it lasts, but on a bicycle path it does not.

According to Robert Perkins at the School of Engineering and Management in the Institute of Northern Engineering at UAF, the difference between pit run or borrow and D-1 is that pit run tends to be rounded and biggish, and D-1 is crushed and smaller, making for little interlocking chunks that are more stable. It's also a difference of about $100 a yard, according to Perkins, so it's a lot cheaper to use pit run. One of the main reasons bike paths fail in Alaska is because of melting permafrost and frost heaving in the spring, so to keep them from breaking up, a good six feet of gravel needs to be laid down.

According to Billy Conner, another UAF engineer, D-1 gravel is stronger, and easier to work with and grade. However, given the load that a bike path will take, the difference is neglible, so long as the fines (silt) content in the pit run gravel doesn't exceed 6% by weight. Where D-1 makes a significant difference, however, is on the highway, where the load includes big
heavy mondo trucks day after day. Conner tells me that DOT did in fact make an experiemental strip of bike path several years ago down by Eagle River using pit run, just to make sure, and there was no discernable difference in the wear between that and bike paths made using D-1.

Conner also told me that the biggest threat to bike paths is the environment. There's really no difference, he said, in resistance to shrubbery and other plants--willows and alders are about equally destructive no matter what sort of gravel is used.

So we'll see just how durable our new bike path is.

Bike paths, by the way, are important enough that, according to the Bicycle and Pedestrian Provisions guide on the Federal Highway Administration's website,
Congress clearly intends for bicyclists and pedestrians to have safe, convenient access to the transportation system and sees every transportation improvement as an opportunity to enhance the safety and convenience of the two modes. "Due consideration" of bicycle and pedestrian needs should include, at a minimum, a presumption that bicyclists and pedestrians will be accommodated in the design of new and improved transportation facilities. In the planning, design, and operation of transportation facilities, bicyclists and pedestrians should be included as a matter of routine, and the decision to not accommodate them should be the exception rather than the rule. There must be exceptional circumstances for denying bicycle and pedestrian access either by prohibition or by designing highways that are incompatible with safe, convenient walking and bicycling....Maintaining access to the transportation system for nonmotorized users is not an optional activity.

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