Impact of 2 + 1 road design on two-lane highways under Brazilian driving conditions

• The work evaluates the impact of 2 + 1 road design on Brazilian two-lane highways. • The paper adapts HCM-7 for Brazilian 2 + 1 highways. • The study recommends optimal passing lane lengths and transition zone types. • A comparative analysis of 2 + 1 highways and two-lane highways with passing lanes is presented. 2 + 1 road design features a continuous three-lane cross section, where the center lane is alternately used by each traffic direction at regular intervals. This layout can be a practical alternative when traffic volumes do not warrant full duplication, environmental limitations restrict construction space, or funding is limited. Although it offers benefits, the 2 + 1 road design remains seldom used in Brazil, primarily because there are few studies examining this highway type that account for local infrastructure features and driver-vehicle dynamics. Therefore, this study aims to assess how the 2 + 1 design affects the operation of two-lane highways under Brazilian driving conditions. The 7th edition of the Highway Capacity Manual (HCM-7) provides a method for estimating the level of service and capacity for 2 + 1 roads. However, recent research suggests that applying the HCM method directly to Brazilian conditions often yields results that do not align with field observations. To address this, the study developed an adapted version of the HCM-7 for Brazilian 2 + 1 highways. The study also offered recommendations on optimal passing lane lengths and suitable transition zone types for various traffic conditions. Additionally, a comparative analysis is presented between the 2 + 1 highway and the two-lane highway with passing lanes, aiming to determine the most suitable traffic and grade conditions for each solution.