In this blog, we will discuss the basic criteria involved in the geometric design of roads, as well as the main characteristics of its four key elements: • The user • The vehicle • Traffic • The road.
We will also explore how the relationships between these elements determine parameters for road design. As the topic can be extensive, we have chosen to divide it to make the reading less tedious. So, let's begin with the USER element.
Regarding their limitations, it is the way road users act, including drivers, cyclists, pedestrians, and passengers, all of which are crucial for a successful geometric project.
The Driver
Generalities of the DRIVER: The driver operates the motor vehicle on the road, and their limitations mainly involve vision, expectation, reaction, and response. Driver's Vision: Vision is the most important limitation for driving, especially visual acuity, peripheral vision, glare, color perception, and depth perception. Visual acuity is the ability to clearly distinguish an object, and even in normal individuals, it has limitations. Approximately 85% of individuals can clearly distinguish objects within a visual cone of 10 degrees. Peripheral vision is the ability to see to the sides of the head. While objects can be perceived in a visual cone of 120° to 160°, when driving, the angle of vision decreases to 100° at a speed of 30 km/h, and only 40° at a speed of 100 km/h. To encompass a wider visual field, such as 180°, drivers need to move their eyes rapidly and harmoniously from side to side, taking 0.5 to 1.3 seconds in daylight at 30 km/h, but artificial light may require a similar time for every 15 km/h increase in speed. Glare is the lack of adaptation to a change in light, usually more intense, such as encountering another vehicle at night in the opposite direction, or when entering a tunnel. Generally, eyes adapt to changes in light by opening or closing the pupil. Pupil adaptation from darkness to light takes about 5 seconds, and it doubles when transitioning from light to darkness. Color perception is usually not a problem for most people, but individuals with congenital color blindness (daltonism) may have difficulty distinguishing colors.
This can be a serious issue when using colors to control traffic with traffic lights, but it can be mitigated by standardizing the position of light in the device, requiring all lights to be visible. Depth perception is the ability to judge the distance to objects and is associated with stereoscopic vision, depending on the correct separation and functioning of the eyes. In the project, deficiencies in this ability are considered, ensuring there are references familiar to most drivers. If not, visibility distances may need to be increased.
Driver's Expectation: It is the predisposition of a driver to respond predictably and successfully to situations, events, and information. When faced with new or unexpected situations, the likelihood of accidents is higher. Therefore, road characteristics should align with the driver's experience, which can be long-term (acquired throughout life), short-term (acquired during a journey), or occasional (associated with events of low probability). Uniformity and consistency of geometric design are essential conditions to meet driver expectations, along with proper traffic signage and regular road surfaces, anti-slip and without potholes.
Driver's Reaction: It is the time it takes for a driver to respond to a stimulus, longer for visual stimuli (0.18 s) than for auditory or tactile stimuli (0.14 s). In traffic situations, the driver's reaction involves four stages: perception, identification, decision, and action. The duration of these stages varies between 0.5 and 2.0 s for simple situations. However, complex situations or uncontrolled variables like inexperience, age, motivation, and risk aversion can increase reaction times up to five times. Nevertheless, a reaction time of 2.5 s is used for project purposes, covering 90% of usual situations.
Driver's Response: It refers to the set of decisions the driver makes while operating a vehicle, resulting from prevailing road conditions. These decisions occur in space and time, and speed relates to these two variables. Higher speeds reduce the visual field, restrict peripheral vision, and limit the time available for drivers to perceive and process information (response). Roads built with an appropriate design speed help compensate for these limitations by simplifying control and guidance activities, providing drivers with adequate information within a clear vision cone, eliminating the need for peripheral vision, and simplifying necessary decisions by separating lanes to reduce information processing demands and provide an appropriate driver response.
The Cyclist
The characteristics of a cyclist are quite different from those of a driver. These characteristics are not considered in the geometric design of roads for safety reasons, and it is recommended not to mix cyclists with motorized vehicles. In fact, the circulation of cyclists on controlled-access roads is usually prohibited. When a significant volume of cyclists is anticipated, specific conditions for safe circulation are provided in adjacent exclusive lanes called cycle paths. Specialized publications should be consulted for design considerations (which will be discussed later in our blog section).
The Pedestrian
The pedestrian is a road user who does not use any vehicle for transportation but travels on foot. The participation of these users on roads should be a primary consideration in planning and design, except on controlled-access roads to which they should not have access. Pedestrians are part of the environment surrounding a road and will be the focus of special attention from the designer. Their presence is naturally more intense in urban areas than in rural areas. To plan and design a pedestrian path correctly, it is necessary to know their main characteristics beforehand. Generally, pedestrians do not walk more than 1.5 km to work or more than 800 m to board a bus. Typically, 80% of their journeys are less than one kilometer.
Pedestrians are less predictable than vehicle drivers. Many of them consider themselves exempt from traffic laws and regulations. Additionally, there is generally little strict enforcement by authorities regarding their behavior on the road, making it difficult to project an orderly and safe pedestrian movement.
Pedestrians tend to follow paths that represent the shortest distance between two points, often resulting in them not walking on sidewalks. Pedestrians resist using overpasses or underpasses when they have to cross a road.
An important factor regarding pedestrian accidents is age. Very young pedestrians disregard traffic due to ignorance, while older individuals pay little attention or adopt challenging attitudes toward drivers. Pedestrian accidents also occur due to the lack of suitable sidewalks, forcing people to share the road with vehicles. Some useful measures to consider for pedestrians in the geometric design include assuming that pedestrians are of advanced age, and therefore: • Consider low pedestrian speeds. • Provide refuge islands. • Provide sufficient lighting. • Use efficient traffic control systems. • Design compatible signage for vehicles and pedestrians. • Locate vehicular and pedestrian traffic lights strategically with large indicators. • Enhance pavement markings.
For sidewalk, stairs, or passenger boarding and alighting areas design, it is of utmost importance to know the width and depth of the human body. Typically, adult males occupy a width of 52 cm and a depth of 33 cm. For project purposes, the human body can be represented by an ellipse with minimum dimensions of 60 cm major axis and 45 cm minor axis. However, larger dimensions are considered for greater comfort. When walking, pedestrian speed varies from 0.8 to 1.8 m/s. For project purposes, a value of 1.2 m/s is usually considered, although this speed decreases as pedestrian traffic density increases.
To characterize the quality of pedestrian flow, a range of service levels has been established from A to F, from the best to the worst. Level A considers an area of 12 m2 per pedestrian, total freedom to choose speed, and no conflicts with other pedestrians. Level F, which would be the most critical, considers an area of up to 0.6 m2 per person; at this level, conflicts are inevitable, and movement is forced and difficult. Procedures for capacity analysis, service levels, and facility design can be found in specialized publications.
The Passenger
The needs of passengers in private vehicles and public transportation vehicles are mentioned in specific points related to them in this and other manuals.
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