From Identifying the incidence of rash, Stevens-Johnson syndrome and toxic
epidermal necrolysis in patients taking lamotrigine: a systematic review of 122
randomized controlled trials :

"Of a total of 4,364 papers regarding lamotrigine, 122 studies met our inclusion and exclusion criteria. In total, 18,698 patients were included with 1,570 (8.3%) of patients experiencing an adverse dermatologic reaction. The incidence of Stevens-Johnson syndrome/toxic epidermal necrolysis was 0.04%."

As stated above, some people may have dermatological reactions, but a much much smaller percentage of those are actually SJS. Nevertheless, most psychiatrists take the cautious road if/when any rash or other skin reaction shows.

From Are the Odds Ever in Your Favor? Car Crashes Versus Other Fatalities | News | Cars.com :

"In 2013, the individual American driver's odds of dying as a result of an injury sustained in an automobile crash (which include pedestrians, bicyclists and motorcyclists involved in car crashes) come out to about 1 in 77 — making it one of the highest-probability causes of death tracked by the CDC."

Getting SJS from lamotrigine: +/- 0.4% chance
vs.
Dying in a car-related accident: +/- 1.3% chance

Based on above, people are three times more likely to die because of a car than to get SJS. Note that SJS, when suspected, can usually be stopped by stopping the cause (i.e. lamotrigine) quickly and getting treated. It's important to always report rashes expeditiously.