UC Davis has recently completed their study on the quality of chip seal and treatments to improve it. You can read the full report here. This report has impacts on our campaign to improve the surface quality of Highway 35/Skyline Boulevard, which was repaved with chip seal in 2013.

The study measured texture and bicyclist ride quality on State Route 1 in San Luis Obispo County and State Route 198 in Monterey County and compared textures to other state routes in San Luis Obispo, Monterey and Mono Counties.

It found that two gradations of the 3/8″ chip seal have been used in state projects, one that is fine and one that is coarser. The report measured macrotexture, which is the surface texture that is most likely to affect ride quality through vibration, through mean profile depth (MPD). While the MPD of pavement surface on California state highways typically ranges from 0.5 mm to 1.5 mm, the median MPD of the finer 3/8″ chip seal gradation was 1.2 mm, while the median MPD of the coarser 3/8″ chip seal gradation was between 1.7 – 3.0 mm.

However, the report also found that five different treatments applied to the existing chip seal were able to produce a lower MPD than the original untreated chip seal section, showing that there are solutions. These treatments are cinder seal, microsurfacing, slurry seal, sand seal and double chip seal.

Bicyclists in the survey considered an “acceptable” level of MPD to be between 2.0 and 2.7 mm where an MPD of 2.0 mm resulted in an average ride quality of 3 on a scale of 1 to 5 (1 being uncomfortable and 5 being comfortable) while an MPD of 2.7 mm resulted in an average rating of about 2 on the same scale. From this, a clear goal would be to have an MPD of less than 2.0 mm to ensure bicyclist comfort.

The report reveals several avenues of improvement for Highway 35 that SVBC can advocate for. If you have any comments on the report, please use the contact box at the bottom of this page (with reference to “chip seal”) or email me directly at emma@bikesiliconvalley.org (please include “chip seal” in the subject line).