to drop back below the freezing level. A final treatment of 350 lbs/lane-mile is also
recommended just after midnight on the 16th to protect the road from refreezing after the
precipitation stops but the road is still considered to be wet.
The warm/in-range, borderline road temperatures were a source of sensitivity for this
storm. The freeze-point threshold of 35F was chosen to represent the temperature at
which the road is reasonable susceptible to freezing. Iowa DOT selected 35 F because (1)
this is a number they were comfortable with given years of using RWIS in-road
temperature sensors and (2) they felt this buffered temperature gave them some level of
confidence that they err on the side of caution. However, the MDSS road temperature
model was proving more accurate during storms in both simulation and live test cases. A
decision was made mid-way through the demonstration to adjust the freeze-point
threshold from 35 F to 33.5 F (closer to the expected +/- 1.5 F error of SNTHERM during
snowstorm events).
If we re-examine the treatment recommendations given a freeze-point threshold of 33.5
F, we find the results shown in Fig. 11.19. Not surprisingly, we see a wider area of warm
road temperatures. The initial treatment recommendation is the same, however the system
has added a plow-only treatment at 1100 on the 15th. This treatment is to be followed by a
300 lbs/lane-mile at 1700 to protect the road at the end of the storm.
Fig. 11.19. March 15-16 forecast timeline with freeze-point adjustment.
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