Road traffic and transportation safety technology (2)

(2) Traffic engineering professional knowledge

1. Sight distance

1) Driving sight distance

Driving sight distance means that in order to ensure driving safety, the driver should be able to see the road ahead at a certain distance on the driving route in order to detect obstacles or When oncoming traffic occurs, measures such as stopping, giving way, missing another vehicle, or overtaking are taken, and the distance traveled by the car in the shortest time necessary to complete these operations. Ensuring sufficient driving sight distance in the plane and longitudinal design of the highway is an important measure to ensure safe and fast driving.

2) Types of sight distances

According to various situations that a car may encounter while driving on the highway, there are five types of sight distances: parking sight distance and passing sight distance. , sight distance for missing vehicles, sight distance for overtaking, and sight distance for obstacle avoidance.

2. Braking distance

The braking distance of a car: refers to the distance required from the driver starting to press the brake pedal to completely stopping. Its calculation formula is as follows:

Where St—— Braking distance, m;

υ, V——Vehicle speed when braking starts, m/s, km/h;

g——Gravity acceleration, 9.8 m /s2;

i——Road longitudinal slope, %;

k——Safety factor of brake.

It can be seen from the above formula that as the vehicle speed increases, the braking distance increases rapidly; at the same time, the adhesion coefficient between the road surface and the tire and the braking distance of the vehicle are directly related. The braking performance and adhesion of the car should be improved as much as possible. For areas with poor road surface conditions or when the weather is cold and covered with thin ice, the vehicle speed must be limited and the distance required for braking should be shortened to ensure driving safety.

3. Linear shape and safety

Whether the composition of road geometric linear elements is reasonable and whether the linear combination is coordinated will have a greater impact on accidents.

(1) Flat curve. Flat curves are closely related to traffic accidents. The larger the curvature, the higher the accident rate. Especially when the curvature is greater than 10, the accident rate increases sharply.

(2) Vertical curve. If the road vertical curve radius is too small, the driver's field of vision will become smaller and the sight distance will become shorter, which will affect the driver's observation and judgment and easily cause accidents.

(3) Slope. According to survey data from the former Soviet Union, the traffic accident rates on plains, hills and mountains are 7%, 18% and 25% respectively. The main reason is that there is no time to brake or brake failure when going downhill.

(4) Linear combination. The reliability of traffic safety is not only related to plane alignment and longitudinal slope, but also has a close relationship with whether the linear combination is coordinated. Although the alignment standards are in compliance with the specifications, poor combination will still lead to an increase in accidents.

4. Intersection characteristics

An intersection occurs when two or more roads with different directions intersect. It can be divided into two categories: plane intersection and grade intersection. Different traffic flows at a graded intersection are spatially separated, so they do not conflict and have little interference with the traffic flow. However, due to the conflict of different traffic flows at a level intersection, it greatly interferes with the normal driving of vehicles and also causes Will lead to traffic accidents. In the existing road network, level intersections are common. Here we mainly introduce the properties of such intersections.

Grade intersections can generally be divided into three categories: one is an intersection without any traffic control, the other is a roundabout intersection, and the third is a signalized intersection with colored lights.

(1) No signalized intersection. Parking methods at unsignalized intersections can be divided into two types: two-sided parking and four-sided parking. Four-sided parking is used at intersections where roads of equal importance intersect. Regardless of priority or non-priority (i.e. main roads and secondary roads), all vehicles must stop and then pass when arriving at the intersection. Two-way parking is usually used when the main road (priority direction) and the secondary arterial road (non-priority direction) intersect. The main road can pass first, and all vehicles on the secondary arterial road will stop and wait for the gap in the traffic flow in the priority direction to pass or merge.

(2) Roundabout. A roundabout is a circular island or island with arcs in the center of an intersection where several roads intersect, so that all vehicles entering the intersection can travel around the island in the same direction. The operation process is generally to merge in different directions first. (Merge), then pass with the same lane one after another (Weave), and finally exit in different directions (Divergence), which can avoid direct intersections, conflicts and large-angle collisions.

Roundabouts have been rarely used recently, so I won’t go into details here. If you are interested, please refer to other materials.

(3) Signalized intersection. The color lights at signalized intersections are composed of red, yellow, and green colors and are used to direct the passage, stop, and left and right turns of vehicles. With the change of the color of the signal lights, the vehicle right of way is transferred from one direction to another. According to the length of the signal cycle And the length of time each signal occupies, the traffic capacity of the intersection can be calculated. Signals are generally installed at intersections with heavy traffic in large and medium-sized cities to control traffic. The traffic capacity of signalized intersections is closely related to signal control design.

The traffic capacity analysis of signalized intersections also includes the analysis of left-turn lanes, straight-left mixed driving lanes, etc., which will not be described here. If you are interested, please refer to the relevant information.

5. Expressway

Expressway generally refers to an annual average day and night traffic volume of more than 25,000 vehicles that can be converted into passenger cars by various automobiles (including motorcycles). It is of particularly important political and economic significance and is specially designed for A highway for cars to travel in separate lanes at high speeds and with all access controlled.

Usually, highway is a term relative to ordinary roads, which refers to roads designed for high-speed driving of cars. It can generally adapt to speeds of 80km/h or higher. It requires smooth lines, small longitudinal slopes, four or more lanes on the road, and a separation zone in the middle. Asphalt concrete or cement concrete is used to pave the road, and guardrails are set up where necessary; to ensure driving safety, complete signs are set up. , marking lines, necessary monitoring facilities and lighting equipment. Pedestrians and non-motorized vehicles are prohibited from driving on the road. When intersecting with railways or other highways, a three-dimensional intersection is adopted. When pedestrians need to cross, they use overpasses or tunnels to pass.