I don't know if SD works differently in Highway Mode, but SD has two timers. One is the "max time" and the other is a "lock out" timer.
On SD, when ALL the conditions required for Higway Mode (or Highway Mode Spark Advance) are met, the leaner Highway Mode AF ratio (and Spark Advance) will kick in up to the maximum amount of time you set (default is 60 seconds on SD). Then the system will reset and redo the cylce...but only after remaining out of Highway Mode (AFR and Spark) for the amount of time set in the Lock Out Timer. I set my Lock Out Timer to Zero, so I remain in Highway Mode continuously as long as I meet the conditions.
There are separate timers for AFR and Spark Advance. I also didn't care for the conditions to set Highway Mode and I "poked the machine code" to test other variables and in differing conditions. EG, Instead of testing the Minimum RPM to enable HM Spark Advance I changed it to Maximum RPM (changed from a Branch Low to a Branch High test) along with adding a test for Minimum MPH which is not part of the code for HM Spark Advance (it is for HM AFR).
Besides changing parameters in the BIN, you CAN also change the actual machine code without even having source code. I use TunerCat TDF Editor to define a constant or table in the TDF to edit the actual machine code if I want to test a "quick and dirty" change. | 
