Probabilistic Model for Design of Freeway Acceleration Speed-Change Lanes

Abstract

A speed-change lane (SCL) is an auxiliary lane added to the traveled way for the acceleration and deceleration of vehicles entering or leaving a roadway. When the length of an acceleration SCL is adequate, drivers are able to accelerate comfortably from the speed at entrance to a speed appropriate to the road, find a gap in the traffic flow, and merge in a safe and secure manner. The length of an SCL is currently determined in terms of the ramp design speed, the freeway design speed, and the acceleration rate. Embedded in these values are assumptions for the operating speed at the entrance and merging points. This study examined a probabilistic approach instead of such a deterministic approach. The main benefit of a probabilistic approach is that traffic flow characteristics are assumed to be stochastic; therefore, the outcome of a probabilistic methodology is a distribution of drivers’ acceleration distance on the SCL. The reliability-based analysis enables designers to select a specific percentile value of this distribution as a design length that better matches a certain situation and avoids unnecessary extra construction costs. This paper presents analytical and simulation models for the application of the reliability approach, with all parameters based on recently collected field data. Even though the presented model should be superior to the deterministic model adopted in current design guides, additional enhancements are recommended for a full reliability-based, safety-explicit design model.

Performance-Based Analysis in Civil Engineering: Overview of Applications

“…Various applications related to geometric design and traffic operations [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] are found in Table A1. The applications rely mostly on the geometric design guides by AASHTO [35] and the Transportation Association of Canada (TAC) [36].…”

“…Ibrahim et al [27] introduced a reliability-based quantitative measure of probability of non-compliance and used it to develop a safety performance function for the relationship between design reliability and expected collision frequency. Probabilistic approaches were also developed for the freeway acceleration distance [28] and freeway speed-change lanes based on acceleration and gap acceptance behavior [29].…”

Reliability Analysis of Intersection Sight Distance at Roundabouts

“…Table 1 presents a summary of previous reliability-based applications on sight distance and related highway geometric design elements. Examples include general geometric design elements by Navin [8], intergreen intervals at traffic signals by Easa [9], ISD for railway crossings by Easa [10], uncontrolled intersections by Easa [11], freeway entrance speed-change lanes by Fatema and Hassan [12] and Hassan et al [13], highway cross sections by Ibrahim et al [14], highway geometric design by Ismail and Sayed [15], truck escape ramps by Greto and Easa [16], and pedestrian green intervals by Easa and Cheng [17]. Most methods have used the first-order second-moment method (FOSM) of reliability analysis, including stop-controlled intersections by Easa and Hussain [18] and left turn at signalized intersections by Hussain and Easa [19].…”

Probabilistic Analysis of Freeway Deceleration Speed-Change Lanes

“…Using such a probabilistic approach can help in providing an assessment of risk and allows for more flexibility in the design. This approach can be particularly useful in the design of the SCL in which the deterministic approach has traditionally used a combination of conservative and nonconservative design parameters (9,10).…”