C172 INSTRUCTOR AND STUDENT VIOLATED LAX CLASS B AIRSPACE.

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AT APPROX XA30 ON 08/SAT/02; ON A NIGHT XCOUNTRY TRAINING FLT FROM SMO TO LGB; MY STUDENT AND I ACCIDENTALLY VIOLATED CLASS B AIRSPACE OVER LAX ARPT BY FOLLOWING WHAT WAS BELIEVED TO BE THE INTERSTATE 10 FREEWAY; BUT IN ACTUALITY; TURNED OUT TO BE THE 405 FREEWAY. SINCE IT WAS MY STUDENT'S FIRST FLT AT NIGHT; I LET HIM FLY THE ACFT TO PRACTICE HIS PILOTAGE SKILLS AT NIGHT WITH THE SIMPLE DIRECTION OF 'FOLLOW THE INTERSTATE 10 FREEWAY;' WHILE I WORKED THE RADIOS AND PLANNED THE APCH TO LGB. NEITHER ONE OF US BEING INTIMATELY FAMILIAR WITH THE LOS ANGELES BASIN AREA; SO I REALLY WAS WORKING WITH THE MAPS AND THE RADIOS IN ORDER TO EXPEDITE OUR ARR INTO LGB. BY THE TIME I RECOGNIZED THAT MY STUDENT HAD TAKEN AN EARLY TURN ON THE INTERSTATE 10 FREEWAY TO FOLLOW A DIFFERENT HWY UNWITTINGLY WE WERE ABOVE THE APCH END OF RWY 25 AT LAX. AT THAT MOMENT; I TOOK OVER THE CTLS; ADDED FULL PWR; AND INITIATED A DSCNT (IN ORDER TO SQUEEZE OUT ADDITIONAL SPD) AND PROCEEDED TO MOVE OUT OF THE IMMEDIATE VICINITY. UPON LOOKING BACK UPON THE SIT; MY DESIRE TO EMPHASIZE PILOTAGE AND DEAD RECKONING AND INDIVIDUAL DECISION MAKING DURING THIS NIGHT FLT CONTRIBUTED TO THIS SIT. ADDITIONALLY; THE DE-EMPHASIS ON ONBOARD GPS UNITS BY INSTRUCTORS TO DISCOURAGE THEIR USE DURING TRAINING SITS IN FAVOR OF LESS EFFECTIVE NAV METHODS. THE PROLIFERATION OF ONBOARD AND HAND HELD GPS UNITS MEANS THAT NAV CAN BE SAFER AND MORE EXACTING.