1. Obtaining a friction value by looking at a road in lieu of testing is taking a chance. There are some surfaces that look wonderful, but can yield remarkably low values due to surface mixture or other variation. Testing should always be the prefered way to go. Aberrant surfaces are extremely rare, but they do exist and need to be identified. I know it is commonly accepted practice to assume a standard coefficent of friction or range, and most often this is fine, but variation does exist. Some new surfaces (like chip-seals) are so rough that I've received > 1G peaks during testing with nearly as high averages.
2. If your testing is done properly, it yields the value or range of value you should use.
3. The best would be to use the involved vehicle (with an accelerometer); next involved vehcile with-out an accelerometer; next an exemplar with similar tires (with /with-out a g-measuring device); next a cruiser (with/with-out a g-measuring device); next a sled; then the charts. Follow the chain to what you can use due to safety; financial; available equipment, etc.
4. +/- 5% is terrific.
5. See if the data is reasonable. If not, decide from there. You may have to re-test.
6. G-analyst. Long history of reasonable results and good correlations with other methods when properly used.
7. Very little. Measurable but not significant. Unless other factors exist, wet + oil build up. Heat such that the road-oil mix is melting, wind that can push the car around.
Ed Phillips EPHILLPW@CO.SAN-DIEGO.CA.US
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