noun
1
BAD : I jumped in and swam towards the child at my fastest speed.
GOOD : I jumped in and swam towards the child as fast as I could.
BAD : The world's population has increased at a very rapid speed.
GOOD : The world's population has increased very rapidly.
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Phases with speed are usually used in connection with vehicles and machines: 'At the time of the crash, the train was travelling at full speed.' 'These cars are capable of very high speeds.'
2
BAD : These new trains can travel in very high speeds.
GOOD : These new trains can travel at very high speeds.
BAD : The car in front of ours continued with the same speed.
GOOD : The car in front of ours continued at the same speed.
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at a particular speed ( NOT with/in ) 'If we continue at this speed, we'll be there in an hour.' 'According to the police report, the car had been travelling at a speed of 110 miles per hour.'
3
DUBIOUS : He always drives at a very fast speed.
GOOD : He always drives at top speed.
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at great/high/top/full/breakneck speed : 'He jumped into the car and drove off at great speed.'