Development has been underway since 1996 of three types of sensor (visible image type sensor, laser radar type sensor, and optical fiber type sensor) that distinguish and track five road surface conditions (dry, wet, water film, snow cover, freezing). In fiscal 2001, performance was verified by detection of 16,000 items of road surface condition data at Nakayama Pass on National Highway No.230 in Hokkaido, where the sensors achieved 94.4% safety integrity level (visible image type sensor). The sensors were improved based on those results, and continued performance verification was conducted at Nakayama Pass in fiscal 2002, while visible image type sensor was subjected to additional performance verification on National Highway No.45 (Miyako Tunnels) in Iwate Prefecture. Safety integrity level of 95.5% was verified in the Miyako testing.

  The performance capability of a single system to process camera images from multiple locations and detect road surface conditions at those locations was verified for the purpose of effectively using visible image type sensor for road management. Testing on National Highway No.45 (Miyako Tunnels) determined how many cameras could be connected in this way.

  The AHS road surface detection sensors are made to meet requirements more stringent than actual current conditions (existing road surface sensors), as shown in the table, because they are used in safety systems. AHS provides information to vehicles from the roadside, and consequently will also distinguish the water film condition, which results in hydroplaning. Provisional safety and reliability target values set for these sensors are 96% or better for safety integrity level and 90% or better for sensing accuracy rate (individual). It is also essential for system safety to have the sensors generate a give up signal when they diagnose themselves as unable to detect (see note).
Note: In the case of visible image type sensor, this would include times when the road surface is not visible because of a standing vehicle, dense fog, etc.

  Field operation tests were conducted in demanding environments at Nakayama Pass on National Highway No.230 and the Miyako Tunnels on National Highway No.45 in fiscal 2002. The results verified stable detection performance and the possibility of simultaneous processing for up to five road surveillance cameras of the type already in place.

  Verification of performance in field operation tests at Nakayama Pass showed that the sensors approximately achieved the provisional target values for individual sensing accuracy rate and safety integrity level, as shown in the figure.

  The features, operating principle, performance, and caveats for application of visible image type sensor are as shown in the figure.

  Particular features of this sensor are that it adopts the visible image camera that is presently in widespread use for road surveillance as its sensor head, and that it has a broad detection range extending to 100 m.

  Field operation tests were held in demanding environments at Nakayama Pass on National Highway No.230 and the Miyako Tunnels on National Highway No.45 in fiscal 2002. The sensors achieved stable detection performance, and the possibility of shortening the initial adjustment period by arranging the databases of cold, snowy regions throughout the country was also confirmed.

  In verification of performance in the field operation tests at Nakayama Pass, the individual sensing accuracy rate and safety integrity level more or less achieved the provisional target values, as shown in the figure.

  The features, operating principle, performance, and caveats for application of laser radar type sensor are as shown in the figure.

  Particular features of this sensor are that it can utilize the sensor module that is in widespread use as a headway sensor on vehicles, so that the sensor head can be made compact and low cost, and that it is capable of measurement in conditions of accumulated snow.

  Field operation tests were held in demanding environments at Nakayama Pass on National Highway No.230 and on National Highway No.25 (Maitani District) in fiscal 2002. The sensors achieved stable detection performance, and it was also confirmed that existing optical fiber used for road temperature detection could be used as road surface sensors.

  In verification of performance in the field operation tests at Nakayama Pass, the individual sensing accuracy rate and safety integrity level more or less achieved the provisional target values, as shown in the figure.

  The features, operating principle, performance, and caveats for application of optical fiber type sensor are as shown in the figure.

  Particular features of this sensor are that it is capable of detection over long distances because it can make use of the optical fiber utilized for road temperature detection, and that it is capable of measuring road temperature distribution in the direction of traffic movement on the road.