Evaluation Of Zero Length Vertical Curves

The objectives of the research were to evaluate the use of zero-length vertical curves with respect to TxDOT design practice, construction results, vehicle dynamics, and accident history, and to compare zero-length vertical curves to minimum design vertical curves (as specified by TxDOT). Researchers determined use of zero-length vertical curves using a questionnaire distributed to TxDOT personnel in all 25 districts. Seventy-one percent of the respondents had used zero-length vertical curves. In this study, 20 zero-length vertical curves and 15 minimum-length vertical curves were evaluated. Constraints limited grade change for the vertical curves to a maximum of two percent. Evaluation included surveying the roadway profile, measuring vertical acceleration, and investigating accident information. The results showed that: sight distance is not applicable to sag curves with grade changes below two percent and is applicable to crest curves with grade changes above 0.5 percent and design speeds over 100 km/h; below 0.5 percent grade change, comfort criteria for zero-length and minimum design vertical curves did not exhibit any practical difference; between 1.0 and 0.5 percent grade change, comfort criteria showed unacceptable performance for high speed tests; zero-length vertical curves were more likely to meet drainage grade requirements than were minimum design vertical curves; and accident studies did not reveal any apparent relationship to the type of vertical curve.