Cambridgeshire Police said they received a call from the driver of the HGV at about 11.30am on Tuesday after he noticed there were people on board and became concerned for their welfare.
Officers went to the lorry, at Haddon services, and found nobody was injured.
Thirteen males and five females, who presented themselves as Eritrean, are currently being interviewed by the police.
The force said the people’s welfare will be handled by the immigration services.
It follows a similar incident on February 17 when three men were rescued from the back of a lorry on the A14 at Brampton Hut Services near Huntingdon in Cambridgeshire.
The men had told police in a call that they were suffocating and struggling to breathe, and officers then located the vehicle which was believed to have travelled from France.
Two of the men presented as Iraqi nationals and the third presented as an Iranian national.
They were taken into custody and their cases will be dealt with according to immigration rules.
A Cambridgeshire Police spokesman said: “Stories like this may be distressing, but highlight that slavery and trafficking are real and police need the public’s help.”
Police have urged people to dial 999 if they see people emerging from a lorry or HGV, if they hear banging from inside a lorry or if they see people entering the back of a lorry.
They said people should be particularly vigilant at service stations or lay-bys, and should raise the alarm if anything about the vehicle does not seem quite right, for example if produce is being thrown from it or something is being done to catch the attention of other motorists.
Anyone who sees any of the above is urged to call police on 999 and to take a note of the vehicle registration number, even if it has foreign plates.