This measure refers to other detection devices which use various types of sensors to detect unauthorised persons in specific areas. Motion detection sensors, RADAR  , LIDAR   or glass fibre technologies can be used detect pedestrian movement and trigger an automatic sound warning (alarm noise or spoken message) by loudspeakers. The aim is influencing the person to modify their behaviour and move to a place of safety. If the detection system is linked with other surveillance services this can facilitate for example the deployment of other security measures (e.g. patrol intervention, interception and prosecution of the trespasser, etc.).

• For suicidal people, the spoken message may have the effect of interrupting the "impulsive mood" of the person.
• Can be used for motion detection before a fence (early detection).
• Can be used in combination with fences, resulting into ’smart’ or ’intelligent’ fencing, which may increase the deterrent effect of a simple fence.

• It is not known how a person that contemplates suicide would react to an alarm sound.
• Be aware that sound warnings may lead to noise pollution which can cause acceptance risks with neighbours and nature conservation organisations. You may encounter aversion and resistance against the system from the people living in the direct environment. May not to be used in rural nature areas because of disturbance of fauna. Communicate before installing. Even after installation, station announcements may not be permitted during some time intervals (e.g., between 23:00 and 07:00) due to issues with noise pollution affecting neighbours. For further details on how to reduce noise pollution you can check http://www.advanced-noise-solutions.co.uk/cs-noise-pollution.html
• Children could play at triggering the sound system as a game.
• These systems do not allow recording of video footage, unless combined with CCTV   for this purpose.

• Qualified staff are needed for maintenance issues or in case human intervention is also deployed when the alarm goes off.

• Overall decrease in trespassing behaviour was experienced over time: 46 trespassing events for the first year, 18 for the second year, and 38 for the third year. 60% drop in trespass from the first to the second year and just a 17% drop in the third year compared to the first. An increase from the second to the third (DaSilva et al., 2006).
• One can expect that an automatic message is less effective than a live message indicating that someone is actually monitoring what is going on, as is the case in other types of incidents such as fires in buildings (Proulx, 1991).
• According to the Master thesis of (Kinder/Spännare, 2012) the payback time on the studied track is relatively short and the probability for a payback time of one year is 64%, two years 90% and three years 98%.
• Speakers in combination with cameras with video detection are in use in Sweden and ProRail is starting a pilot study in The Netherlands in quarter 3 -2012.
• Linking CCTV   with public address-system-announcements may be effective in deterring children but less effective for adults (RSSB  , 2005).
• Deters vandalism (RSSB  , 2005).