Speed Harmony for Safer Highway
Speed and Its Impact on Highway Safety
Speed is one of the most important causative factors in severe crashes and accidents on highways, leading to severe injuries or even fatalities. Higher speed is a major contributor to severe crashes as drivers have less reaction time to respond to sudden adversities, making it harder to stop or maneuver quickly to avoid accidents. At higher speeds, the safety features installed in the vehicle system become less effective. The impact force during a collision at higher speed is significantly increased, leading to more severe consequences for the occupants of the vehicle.
Pedestrians, cyclists, and motorcyclists, who constitute the most vulnerable road users, have a very high risk of fatality if hit by a speeding vehicle. Studies have established that vulnerable road users are unlikely to survive if struck by a vehicle traveling at 50 kmph or more.
Design Speed and Its Role in Roadway Design
Design speed is used in the geometric design of various elements, including road alignment. Proper land use planning and studying community livability, incorporating the needs of all road users (pedestrians, motorcyclists, heavy trucks, etc.), enable transport planners to select an appropriate design speed for a road. The design speed is chosen to conform to topography, adjacent land use, and the functional use of the highway. It should also align with the habits of the majority of drivers.
However, it has been observed that the concept of design speed does not always deliver a consistent alignment. Highway safety is achieved conservatively using design speed, making it suitable only under reasonable conditions. The design speed is considered safe from a designer’s point of view, but faster driving vehicle users often do not adhere to it. Drivers tend to select their speed based on the road environment.
In many countries, such as Sweden, Switzerland, and Germany, the design speed is selected as the 85th percentile speed for consistency. According to the NCHRP report 502, design consistency means that the design conforms to highway geometric and operational features and meets drivers’ expectations. While dealing with design consistency, factors such as the difference between operating speed and design speed and speed reduction between successive geometric features must be considered.
Design Speed and Traffic Flow Considerations
Design speed is used for individual geometric elements such as:
- Sight distance
- Radius of curves
- Road friction
- Topography
- Road function
It is not applied to entire road sections. To ensure user safety on highways, traffic flow quality and composition (percentage of passenger cars, trucks, and heavy trucks) must be assessed and quantified properly. For safe highway design, speed differences among vehicles should not exceed 15 kmph. A higher number of heavy vehicles can result in speed non-uniformity, making a lower design speed desirable. Additionally, the presence of pedestrians, cyclists, schools, and residential areas discourages higher design speeds.
The length of crest and sag curves is determined by the design speed and is based on traffic volume and functional road classification.
Operating Speed and Its Impact on Highway Safety
Drivers often operate their vehicles at a speed different from the design speed during free flow. This is referred to as operating speed, usually the 85th percentile speed of observed speeds. Discrepancies between design speed and operating speed are unfavorable for highways. Safety features, including the clear zone (an unobstructed flat area beyond the travel way for errant vehicle recovery), are determined based on design speed rather than actual operating speed. A wider clear zone enhances the safety of errant vehicles.
Operating speed depends on multiple factors, including:
- Driver behavior
- Environment
- Vehicular characteristics
- Road features
Operating speed is fundamental to the roadway and is used in determining design speed for selecting appropriate design elements. Each road system consists of different road functions, and variations in topography must be accounted for in good design. Studies indicate that increasing the curve radius and the length of horizontal curves increases operating speed. Additionally, lane width, road width, and right shoulder width affect operating speed.
To minimize the difference between operating and design speed, geometric alignment should be revised based on preliminary design speed alignment. Driver behavior and speed selection depend on road geometry and other features. Inconsistent design may lead drivers to adopt inappropriate speeds, increasing the risk of crashes. Hence, there must be design consistency for all road types and topographies.
Criteria for Checking Design Consistency
The three important criteria for checking the design consistency of a road are:
- Checking the difference between operating speed and design speed.
- Checking speed consistency across successive road sections.
- Assessing side friction adequacy.
The relationship between speed, crash severity, and geometric road features must be properly ascertained.
Posted Speed Limits
Posted speed limits represent the highest safe and reasonable speed under favorable road, traffic, and weather conditions. These speed limits are determined by law and displayed on speed limit signs. The posted speed limit is a function of design speed. It can be increased by either increasing the design speed or allowing the speed limit to exceed the design speed.
The 85th percentile speed is the speed at which 85% of vehicles during free-flow conditions travel at or below that speed. This speed is rounded to a value divisible by 5 to determine the posted speed limit.
When designing a new road or road section, care must be taken to accommodate operating speeds consistent with the projected highest posted speed limit based on the roadway’s functional characteristics.
