TMC Aler C标准：交通事故解决方案与RDS-TMC编码指南

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TMC Aler C标准，全称为"Traffic Message Channel Alert Coding Handbook"，是一项针对汽车导航系统（RDS-TMC）中交通消息广播的编码协议。该标准由ENV 12313-1定义，主要关注交通事件的实时通知，如路况警告，对驾驶员至关重要。标准的核心内容是Alert-C编码规则，它提供了如何将这些交通信息有效地转化为无线电信号以便车辆接收和解析。 TMC Aler C版本F02.1于1999年1月2日发布，包含了多个修订版的历史记录，如F02.1、F02和F01，反映了标准的持续更新和完善。这份手册旨在补充标准本身，因为它不仅列出编码规则，还提供了实际操作中的示例，解释了规则背后的逻辑和原因，以及如何利用可调参数来优化系统的性能。因此，这份文档对于以下几类读者尤其有用： 1. 程序员：他们需要深入了解如何编写和实现符合TMC Aler C标准的交通消息代码，以便开发出能够准确传达实时交通信息的车载信息系统。 2. 车载电子设备制造商：他们需要确保其产品能够兼容并正确处理Alert-C编码，以提供无缝的驾驶辅助体验。 3. 交通信息服务提供商：他们负责生成和发送交通信息，需要知道如何优化编码策略以提高信息的准确性和效率。 4. 汽车工程师和系统集成者：他们需要在设计车辆通信系统时考虑Alert-C标准，确保车辆能及时获取并处理必要的道路状况信息。 5. 研究人员和标准化机构：这份手册可以帮助他们理解标准制定的背景和目的，为后续技术发展提供参考。 TMC Aler C标准和Coding Handbook是一个关键资源，对于理解和应用在汽车导航和通信系统中的交通消息编码至关重要，它促进了道路安全和驾驶者便利性的提升。通过深入学习和遵循这个标准，汽车行业参与者可以确保他们的产品和服务能够满足现代驾驶环境的需求。

TMC Compendium - Alert-C Coding Handbook

For some events it will be necessary to use a point location with no extent (e.g.: exit

closed). In these cases the direction bit will still be used to indicate the direction in

which the exit is closed (i.e.: there is a difference between extent +0 and -0). If it is

required that the direction bit should have no effect, it is necessary to use either a bi-

directional event (like “dense fog”) or use a special control code to switch the event

from directional to bi-directional. Using the example above (“exit closed”), changing

to a bi-directional event would result in a message indicating that both directions are

affected. Note that for areas, extent and direction are always ignored.

To make stepping based on direction possible, a location database is needed.

2.3.3 Location Database

The basic tool to code the “Where” is the location database. A location database can

contain different location types. The standard ENV 12313-3 (Ref. 3) defines the

following classification scheme.

Each location is described by a type code which is composed of:

• A character (A, L or P), indicating the category (Area, Line or Point).

• A number indicating the type.

• A dot.

• A number indicating a subtype.

Example:

P1.8 - roundabout (P: category = point, 1: Type = junction, 8: subtype =

roundabout)

Each of the three main categories (Area, Line, Point) may represent one or more

types and subtypes. Annex 3 gives a quick overview of the different, currently

defined, locations types to show the large number already available (as these may be

extended, only use Ref. 3 as a reference).

Each location is assigned a 16 bit code, allowing more than 60 000 locations in one

database. In addition to location code, location type code and location name,

additional information is given for each location. What additional references and

attributes are given depends on the location type code. Annex 4 shows the possible

combinations. For the following chapters, it is only the following columns that are of

importance:

• First Name, Second Name and Road Number describe the location itself.

TMC Compendium - Alert-C Coding Handbook

• Positive Offset, Negative Offset, Line and Area Reference describe the

relationship with other locations inside the location database.

The First Name is normally the name of the location. However, in the case of roads

and segments it is the name of the segment start. So, for the motorway between

Rotterdam and Amsterdam, Rotterdam would be First Name, while Amsterdam

would be placed into the Second Name field.

Road Number is just what is says, it is the number of the road. In several countries

this is not only a number but also a letter to characterise the Road type. Typical

Road Numbers are M5, A4, 128, B49n, ST1027.

The Positive and Negative Offset contain the location codes of the following and

preceding location. They only make sense with point locations that are part of a

segment and segments that are part of a road.

The Line Reference identifies on which segment a given Point is situated (or on

which road a given segment is situated).

The Area Reference identifies which next bigger area the location belongs to. For

segments and roads this is the smallest area that just contains all the points belonging

to the segment or road (so for most motorways this will be the location code of the

country itself).

All of this referencing is well described in the Location Coding handbook (Ref. 6).

To give a precise representation of where a given situation occurs, it is always

necessary to access several of these fields in the location database.

Example:

In case of traffic information, the location may be a point location on a road. This

only identifies the so called “primary location”. For the coding of “where”,

more locations of the location database must be identified:

Next it is necessary to identify the so called “secondary location”. This is normally

chosen in such a way that the event is now bracketed by the two locations. The

second location is not transmitted directly, instead it is used to calculate how many

steps this second location is away from the primary location. The number of steps is

the extent. The calculation is performed by following the Offset References, either

in a positive or negative direction, starting at the primary location and going through

to the secondary location. (Note: the number of steps from secondary to primary is

TMC Compendium - Alert-C Coding Handbook

not always identical to this number as there may be locations present for only one

direction!)

If the Offset Negative Column is used, the direction bit is set to negative (1); if the

Offset Positive Column is used, the direction bit is set to positive (0).

The total extent and its direction is identified by means of the direction bit, the

extent and (possibly) the control codes that have been used to enlarge the extent.

The receiver is now able to step from location to location in the direction indicated

by the negative or positive direction bit to identify the secondary location.

The simplicity of the calculation is misleading; in the real world this process is quite

difficult to implement because the coder has to look at the following rules during the

process:

• The points/segments may not be on different roads.

• The number of steps between them (= extent) may not exceed 31.

If, for example, the primary and secondary location code are known, it is still not

clear in which direction the system has to search for the secondary location (to

determine the number of steps). One solution is to enter the locations in two steps:

• First, the primary location is entered.

• The system then calculates, on the fly, all possible secondary locations (up to +-

31 steps in each direction), this list is displayed to the user for entering the

secondary location.

This ensures that the two rules are enforced but the system has to do a lot of

calculations in the database. Pre-calculated secondary locations can solve this

problem but only as long as the database is small.

The coding process finishes here. Note that the Area and Line References were not

needed, they are only used in the decoding process. However, as most systems will

be requested to display the message in the same format as it is presented in the

receiver, most coders will also have to perform the functions of the decoder.

In the receiver, the decoder is able to rebuild the message. It gets:

• The Road Number and Segment names from the Line Reference.

• The area names from the Area Reference.

• The types from the Location Type code.

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