Surface Treatment Macrotexture and Bicycle Ride Quality

This memorandum summarizes the results of measurements of macrotexture on a set of Caltrans and local government pavement surface treatments, and the results of bicycle vibration measurements and a survey of bicycle ride quality on most of those sections. The work was performed to address concerns raised by local bicyclists regarding ride quality after a modified-binder seal coat (chip seal) was placed on State Route 1 in San Luis Obispo County (SLO-1). The test sections that were used for the bicycle vibration and ride quality measurements included various surface treatments on existing chip seals on State Route 198 in Monterey County (Mon-198), several locations on SLO-1, and on other state highway and county road sections in Yolo, Butte, El Dorado, Placer, Sonoma, Marin, and San Mateo counties. Macrotexture was measured in terms of mean profile depth (MPD). Three different test methods were used to measure MPD: the sand patch method and the laser texture scanner (LTS), which provide measurements on a small area at a single location (about 12 square inches [100 square cm]); and the inertial profiler (IP), which is mounted on a vehicle and provides a continuous measurement in the longitudinal direction. Also presented are statistical correlations between macrotexture, roughness (in terms of International Roughness Index [IRI]), bicycle vibration, and bicycle ride quality for the initial set of treatment sections surveyed, and modeling of the relationships between macrotexture, roughness, vibration, and perceived ride quality for bicyclists. Conclusions are presented regarding the MPD values measured on various road sections, including those built with two 3/8" aggregate gradation chip seal specifications (one coarser than the other), and the variability of MPD found between sections built using the coarser 3/8" aggregate grading. Conclusions are also presented regarding the three MPD measurement methods. Findings are presented regarding the MPD values from other types of treatments, including possible treatments to be placed on SLO-1 to address a bicycle ride quality issue. Results are also presented regarding the effects of trafficking on MPD values. Correlations between bicycle vibration and MPD are shown, and between MPD, IRI, bicycle vibration, and bicyclists’ perception of ride quality and pavement acceptability for use by bicycles. Recommendations are made regarding a range of MPD values that might be used to select chip seal specifications based on “acceptability” where bicycle ride quality is an issue, selection of remedial treatments for SLO-1 based on the Mon-198 test sections, naming conventions for chip seals, and the use of additional rolling to reduce the texture of chip seals built using the coarser 3/8" aggregate grading.