Complex sentence: He deemed the disentanglement of the traffic stream and the massive expansion of the means of public transport essential in order to realize the ideal of a garden city, however in his opinion the city was and would always be too small for the underground.
Split into simpler sentences: He deemed the disentanglement of the traffic stream and the massive expansion of public transport essential in order to realize the ideal of a garden city. However, in his opinion the city was and would always be too small for the underground.