There's some rather fabulous FOI requests on public record.
For instance.

"I would like to know when the Scottish Government
decided to put Gaelic on all of the road signs."

All?

(the answer was 2002 as Trunk Roads policy for Argyll & Bute and highland)

"I would like to know how much it has cost the Scottish Government to change road signs from English to include the Gaelic... since the Scottish National Party have been in power."

The answer was £750k over 3 years on a replacement for maintenance basis

"I would also like to know how many complaints that the Scottish Government has received about Gaelic on road signs since the Scottish Government decided to install Gaelic road signs."

Must have been crushing to get the answer of "2".

"I shall be obliged if you could advise me the cost of the aforementioned signs since local government reorganisation in 1996."

The polite answer is "Transport Scotland don't control LA roads budgets, but here's the breakdown of 3 years worth of spending on Gaelic by us"

"Transport Scotland maintain a membership with Gaelic Place Names [AAA], who provide the organisation with researched and proofed place name translations. This membership is paid at a rate of £2,000 per annum."

"Transport Scotland has directly spent the following on Gaelic signage or material in the past three years:
Gaelic identity work - £3,476.15
Gaelic Translation – "A seatbelt can save a child's life. And it's the law" - £34.50
Total: £3,510.65."

"For Rail, £1,646.25 (in 2010) was spent relating to trademark protection of several Gaelic elements. The introduction of Gaelic signage at ScotRail stations was part of the overall ScotRail rebranding programme which was included at no extra cost "

"Transport Scotland does plan to introduce bilingual signs on the A9 as part of the dualling process... on the basis of opportunities created by programmed improvement works or planned maintenance."
(that's the polite way of saying "we were spending it regardless)

"The £5 million A82 Crianlarich Bypass opened in December 2014 included an approximate spend of £115k on signage, 44% included Gaelic text alongside the English text"

From 2019.
"Please provide details of all complaints received by Transport Scotland relating to the signs since their introduction began in 2002."

Eleven.
In 17 years.
(Half of them complaining that the Gaelic spelling was wrong and politely correcting it)

It must be utterly bizarre to live in this dream world where you've imagined that THENATS are spending £30 million a year abolishing English from road signs only to find out that the actual ongoing costs are effectively zero, a rounding error in Transport Scotland's tarmac budget

It also must be very disappointing to find out that 67% more was spent under the Labour and Lib Dem administration on trunk road bilingual signage than by the SNP.

The Transport Research Laboratory did a very big study on bilingual signs back in 2012, paid for by Transport Scotland (cost £160k over 3 years). It's a rather long read (210 pages) with two rather short conclusions.

To paraphrase.
There's anecdotal evidence that bilingual signs take marginally longer to read and could increase driver workload but increased risk is rare.
"There is evidence from the survey... that drivers may be managing this extra demand through a slight reduction in speed."

"It is concluded... while there is evidence that bilingual signs may have increased the demand of the driving
task, that this increase can be absorbed and managed by the driver and therefore does not result in a significant increase in crash risk and accident involvement. "

Amusingly, 9% of respondants to the TRLs survey who had Gaelic put them straight that the English made the signs harder to read

And nearly all respondants without Gaelic said that they only read the English bit anyway...

Which kind of contradicts the "I can't understand the sign" bit, if people admit they aren't reading it anyway.

If you can't find your way to Fort William using these signs, you probably should not be in charge of a motor vehicle.