Mae tystiolaeth ryngwladol o gysylltiadau cryf rhwng gostwng cyflymder a lleihau nifer y gwrthdrawiadau a phobl sy’n cael eu lladd neu eu hanafu o’u herwydd. Er enghraifft, mae ymchwil a wnaed gan y Labordy Ymchwil Trafnidiaeth (Transport Research Laboratory) yn 2000 yn dangos, ar gyfer ffyrdd trefol â chyflymder cyfartalog isel, fod gostyngiad cyfartalog o 6% mewn gwrthdrawiadau gyda phob gostyngiad o 1mya mewn cyflymder cyfartalog.
There is international evidence of strong links between lowering speeds and decreasing the number of collisions and people killed or injured from them. For example, research undertaken by the Transport Research Laboratory in 2000 shows that for urban roads with low average speeds, there is an average 6% reduction in collisions with each 1mph reduction in average speed.