To hitch from Peterborough is possible, but you have to go some way out of the city. Although I have hitched from the train station.
If heading away toward A15 and Sleaford. you need to find a service station. There are 2 possible:
Shell. This is slightly better as just after there is a bus layby that cars can pull into. Its just after a little roundabout. But about 2km from the bus station so taking the bus there is possible.
If continuing toward Lincoln you will pass small towns of
Bourne - toward Peterborough there is a bus stop opposite the church and this has been good in the past.
Sleaford - from Bourne outside the Esso service station there is a little roundabout. Just after is a crossing and here the cars must slow. Hitch from here and cars can pull up just after St Davids Lane. Don't go to Morton its really close.