Please help with the understanding of the following sentences.
It was only when the traffic dropped behind him, and the Roman directness of the arterial highway stretched uneventful in front of him that his mind went past Liz to the thing her image was hiding: Searle. Searle. Poor Serge's 'middle-west Lucifer'. Why Lucifer, he wondered? Lucifer, Prince of the Morning. He had always pictured Lucifer as a magnificent, burning figure six-and-a-half feet tall. Not at all like Searle. What in Searle had suggested Lucifer to Ratoff's accusing mind? (Chapter 5 To Love and Be Wise, http://gutenberg.net.au/ebooks08/0800551h.html#ch05)
Q1: Does "Roman directness' refer to the direction of the arterial highway?
Q2: What are 'magnificent' and 'burining' in the paragraph? Since is has been specifically indicated of Lucifer's height, does magnificnet mean handsome in stead of grand?

Question

Answer

The Roman's were known for creating excellent roads throughout Europe to move armies, so probably refers to it being straight and made for long-distance traveling. 
Magnificent isn't a word people would normally use to describe the devil, since it's a positive term; my best guess is it means "impressive", possibly overly muscled.
The devil and hell are often portrayed surrounded by fire and unending flames, hence "burning" is very appropriate.

Answer

1. The Romans occupied Britain for four centuries (43 - 416 A.D.) and built a national network of straight and direct highways for moving troops and military supplies.  "Directness" means the highway is very straight. "Uneventful" means there are no bends.

2.  The word Lucifer is taken from Latin, which means "the morning star", or, "light-bringing".  Lucifer before his fall from grace can be magnificent.  William Blake's drawings of Lucifer show some beautiful muscular nude bodies.  Other drawings through the centuries have shown Lucifer to be a tall, good-looking, sexy man.  Lucifer is also known as "son of the dawn", hence Prince of the Morning.

So you can now see the historical and cultural background to the answers to your two questions.