Mae'r dystiolaeth yn glir – mae gostwng terfynau cyflymder yn lleihau gwrthdrawiadau ac yn achub bywydau. Mae gwaith ymchwil blaenorol (Sefyllfa’r dystiolaeth ar derfynau cyflymder 20mya mewn perthynas â’r effeithiau ar ddiogelwch ar y ffyrdd, teithio llesol a llygredd aer: Awst 2018) wedi dangos bod 40% yn llai o wrthdrawiadau mewn ardaloedd 20mya, o'u cymharu ag ardaloedd 30mya. Yng Nghymru, drwy gyflwyno 20mya yn eang, amcangyfrifwyd y byddai 6 i 10 o fywydau'n cael eu hachub a 1200 i 2000 o anafusion yn cael eu hosgoi bob blwyddyn (Twenty miles per hour speed limits: a sustainable solution to public health problems in Wales). Mae atal yr anafusion hyn yn werth amcangyfrifir £92miliwn bob blwyddyn.
The evidence is clear, decreasing speed limits reduces collisions and saves lives. Previous research (The state of the evidence on 20mph speed limits with regards to road safety, active travel and air pollution impacts: August 2018) has shown that there are 40% fewer collisions in areas with 20mph compared with 30mph. In Wales, it has been estimated (Twenty miles per hour speed limits: a sustainable solution to public health problems in Wales) that with widespread introduction of 20mph, 6 to 10 lives would be saved and 1200 to 2000 casualties avoided each year. The value of preventing these casualties is estimated to be £92m each year.