The CPU is the least of your difficulties if you're making a safety-related project with moving vehicles. Honestly, tread into this area with great caution.
The caveat at the botttom of the datasheet is standard across Atmel (and extremely similar across all manufacturers), and actually reads "Unless specifically provided otherwise, Atmel products are not suitable for, and shall not be used in, automotive applications." (My emphasis). Your lawyer will tell you whether being sold as "automotive grade" counts as "specifically provided".
Automotive parts have different part numbers, see the ordering information of automotive data sheet:
I couldn't find any ATMega48-15AT for sale, but you can see 328PB parts very clearly marked Automotive (at Atmel's sales web site):
You might be interested in the answers to Is "automotive" grade always better? which describes some of the ways "automotive grade" differ from the ordinary ones. In brief, it depends on the part.
My understanding for the ATMega CPUs is that amongst other things they are essentially the same but have a slightly reduced nominal specification (clock rates etc) in order to qualify adequately on reliability across the temperature range. ("If you use it more gently we will make a stronger promise about reliability.") They may or may not have more QA during manufacturer; they are likely to have better batch traceability etc.
The "automotive grade" merely means the manufacturer has done a certain amount of testing and promising against automotive industry standards. I believe it's common to use non-marked parts and do the testing yourself. And of course there is an awful lot of system-level testing and proving.
Having said all that, use the simplest possible parts, as conservatively as possible, with as much safety mechanism as you can. In comments you say you want a "MCU that will be guaranteed to not have random reboots or hardware glitches". No one will give you that guarantee. In any case, these problems have almost entirely board-level, not CPU-level causes. Make sure you have good reliable power, take very good care over the clock generation, use the brown-out detector, watchdog etc. Make sure every input and output is failsafe etc. Audit any libraries as carefully as your own code: if your application is simple enough, consider avoiding libraries altogether.