Five Axles 
Copyright 2015 by Paul Niquette. All rights reserved.

Are you puzzled by this photograph?

A certain systems engineer in California took his annual vacations in Brittany, where he was puzzled by two things that he rarely saw during his travels through the countryside: 18-wheel trucks and damaged highways.  A mere coincidence, one might suppose.  Or not.
Brittany is famous for its transportation infrastructure, wherein a network of routes communales, departmentales, and nationales seem relatively unblemished by crocodile cracks, potholes, tar-filled seams, and crooked paving segments compared to highways, freeways, and roads in California
In 2012, that systems engineer retired and now lives in Brittany, where he often sees street sweepers and foliage trimming machines, but as for giant repaving projects?  Not so much.  He became puzzled by that.

B-RigHeavy trucks damage pavements. That's an established fact.  One might suppose that French camions must not be all that heavy.  However in California the weight-limit for a five-axle C-rig is 40 tons (80,000 lbs); in Brittany the limit for a five-axle B-rig is 38 Tonnes (84,000 lbs).  Hmm.

This puzzle assumes that technologies for regional roadway construction are comparable.  The title Five Axles gives recognition to a feature of half the rigs in the truck-fleets in both Brittany and California.  Solvers are invited to observe in the sketch below that the wheels on a B-rig are configured differently from those on a C-rig and that the B-rig carries comparable loads on 12 wheels -- 50% more weight-per-wheel than the C-rig... 
  Outline Comparison
Maybe the difference in how truck wheels are laid out explains why pavements in Brittany are as smooth as a billiards table and the right-lanes on California highways ride like practice courses for the Baja Badlands Race. 

 What is your explanation?

  Solvers might also want to come up with the reason for why one of the axles on the B-rig is retracted.