Wow, roadside monuments! Probably my favourite conversation point!
To keep this short I will only say ones that have not yet been mentioned.
The 'Peas' bridge on the M25
The tower of light on the M5 near weston-super-mare
The wicker deer at Newtown on the A303
The wall with loads of gnomes on the A30 in cornwall
The wicker sculpture at the exeter inn on the A369
The orange elephant on the A38 at Kennford, abstract sculpture at bristol airport and the wooden dog also on the A38
The various large sculptures of deer etc on the A35 (a bit like some of those that have already been mentioned)
The metal swan, family of bears and the telephone box with a clock attached to it on the A3052 in Devon
The animal topiary on the A377