Curiously, we have seen this question asked and answered more anecdotally than directly by the automotive community. Its frankly a safety issue, and thus deserves more attention than it has received. So, the answer is no, do NOT use speed, or cruise, control in the rain. We will, however, delve a bit deeper.
We searched the National Highway Traffic Safety Administration (NHTSA) and were surprised to find nothing. Consumer Reports, in a single paragraph, makes mentions turning cruise control off in the article linked here about driving in downpours, but says nothing else anywhere else.
Something called the National Safety Commission, which we’ve never heard of, had more to say on the subject, also concluding that cruise control should be off. We also found a reference to truckers being taught to leave it off in wet conditions. The only information found from an official, read government, site comes from the state of Maine, and their message is leave it off.
basic problem is that with the cruise control on, hitting a puddle or standing water can cause the tires to spin faster, if only for a moment. The result can be hydroplaning. Cruise control alone simply cannot compensate for this. The natural driver’s instinct to brake at that moment would disengage the cruise control, but can exacerbate the slide. All of which might have been avoided with the cruise control been left off.
Drivers who have lost control in wet conditions using cruise control report their vehicles accelerated when control was lost. This is simply not possible, and likely an illusion or impression created by the loss of traction.
To be sure, the advanced nature of today’s vehicles begins to blur the lines. Stability control could, and we emphasize could, compensate for this. However, we have simply been unable to find any definitive research one way or the other. The move towards autonomous vehicles certainly suggests that all weather conditions must be accounted for, but testing in adverse weather is only beginning.
For the time being, we conclude that it is best to play it safe and keep cruise control off if the roads you are travelling are not perfectly dry.