Rainy-weather speed limit strategies on highways with variable longitudinal slopes

Area of Science:

• Traffic Safety Engineering
• Vehicle Dynamics
• Environmental Impact on Transportation

Background:

• Rainfall significantly impairs highway safety by reducing pavement friction and driver perception.
• Adverse effects of longitudinal grade on vehicle dynamics are amplified during rainfall.
• Current speed management strategies often lack dynamic adaptation to rainy conditions.

Purpose of the Study:

• To develop a dynamic safe speed-limit model for rainy weather.
• To account for pavement friction, longitudinal slope, and perception distance in speed management.
• To improve car-following behavior and speed adaptation models for adverse weather.

Main Methods:

• Development of a dynamic safe speed-limit model.
• Improvement of the Intelligent Driver Model (IDM) for rainy conditions.
• Simulation experiments on a highway section under varying rainfall and slope conditions.

Main Results:

• Model validation showed high consistency with observed speeds (RMSE 2.13-3.08 km/h).
• The proposed speed-limit strategy reduced conflict rates by up to 82.4%.
• Increased rainfall intensity and downhill slopes significantly narrowed safety margins.

Conclusions:

• Progressively stricter speed limits are needed for rainfall intensities above 1.0 mm/min.
• Downhill sections (-3% slope) are particularly critical during rainfall.
• The findings provide a quantitative basis for adaptive highway speed control in rain.