No real infrastructure investigation. The roads leading into and around Knebworth are constantly congested and the smaller roads used as rat runs.
Also there is more heavy traffic using the already over used bridges!
Any kind of development without the infrastructure already in place will bring Knebworth to a halt, full stop.It just doesn't have the capacity!!
The local employment will not benefit, as most of the residents will have to commute, therefore causing more chaos.

Then there is the greenbelt, local wildlife, conservation areas, etc.

The real solution is to build another Garden City development, as people have previously mentioned.