Abstract
Road geometric design is a key factor impacting driving safety and efficiency. In highway profile design, speed reduction is used to determine critical length of grade. Previous research generally concentrated on the relationship between speed reduction and crash involvement rate to establish the recommended value. Limited research results have been reported at this point concerning speed reduction and traffic efficiency. This study aims to fill the gap by investigating tolerable speed reduction with different vertical slopes considering traffic efficiency. Firstly, appropriate experimental sections were determined after field survey. Traffic data including vehicle count, timely speed, vehicle type, and headway time were then collected on an expressway in Shaanxi Province. The associated traffic efficiency was derived from traffic volume and average speed. After this, the modeling between speed reduction and traffic efficiency was processed with different slopes. The correlation between speed reduction and traffic efficiency was therefore verified. Finally, the prediction model of optimum speed reduction concerning traffic efficiency under different vertical slopes was introduced. It was found that the critical length of grade can be longer with traffic efficiency as the major design control incorporated with slopes of 3–3.5%. The existing regulation in critical length of grade at 3.5–5% can benefit both safety and efficiency. The findings can provide a reference for vertical alignment design, leading to high-efficiency road systems.
Highlights
The rapid increase in traffic volume has led to traffic jams on expressways, affecting driving safety and efficiency [1]
Varying vertical slopes for the vertical alignment of road geometry were included in this research
The results demonstrated the optimum speed reduction for the investigated vertical slopes and provided a speed reduction prediction equation with the slopes of 1–5%
Summary
The rapid increase in traffic volume has led to traffic jams on expressways, affecting driving safety and efficiency [1]. China is a mountainous country, and the profile designs of its roads need to overcome the country’s severe terrain, complicating vertical alignment [2]. Road geometry has great influence on road safety and traffic efficiency [3]. On uphill sections of mountainous expressways, heavy vehicles need to decrease their speed to overcome greater slope resistance. The mixed traffic of passenger and freight vehicles with large speed differences can reduce the driving freedom of overtaking, further resulting in lower traffic efficiency, especially on four-lane freeways [4]. It is of great importance to study the vehicle driving performance on uphill sections of mountainous freeways for relieving traffic pressure and safe driving. The study of speed reduction in relation to traffic efficiency has important practical significance on uphill sections of mountainous freeways
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