SSD
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SSD, sometimes FSSD, stands for (Forward) Stopping Sight Distance.
This is measured to determine the safe design speed for a road as a road user must be able to stop within a particular distance when travelling at a given speed.
The formula to determine is usually given as:
SSD = vt+v^2/2(d+0.1a) where: v = speed (m/s) t = driver perception-reaction time (seconds) d = deceleration (m/s^2) a = longitudinal gradient (%); + for upgrades and - for downgrades
Further calculation criteria including specific values are available in Manual for Streets 2, but as a general rule any SSD calculation exceeding 60km/h design speeds should instead follow guidance in CD 109 of the DMRB.