The death threats were made on the bulletin board of a website for sports-car enthusiasts.
Sustrans press officer Kevin Saunders said he hopes the threats were just by "immature fanatics getting carried
away," but worries "this is indicative of a more general attitude in the UK that refuses to accept that speeding is a criminal offence that needs to be enforced."
Saunders said Sustrans "fully supports measures to enforce slower speeds such as speed cameras and lower speed limits, especially in residential areas and around schools.
"Far too many drivers in the UK seem to take the attitude that speed limits are merely a guide, and that speeding
is a minor misdemeanour, but the figures clearly show that speeding is responsible for thousands of unnecessary deaths and serious injuries every year.
"Sustrans considers it important to support the police force in its speed camera initiatives and has no sympathy for drivers who break the law in a machine that is a potential deadly weapon."