This study compares the Lucerne German allophones of Middle High German <l> using data from the crowdsourcing smartphone app Dialäkt Äpp, to those of the Linguistic Atlas of German-Speaking Switzerland. In doing so we examine language change in real-time across c. 70 years. In the north of the Canton of Lucerne, use of /l/-vocalisation has receded, whereas in the west, and in and around the city of Lucerne, /l/-vocalisation occurs more frequently than at the time of the historic atlas. Both age and sex differences were found to be statistically non-significant, though older speakers tended to vocalise more. Generally, however, we report substantial areal heterogeneity which we suspect is due to the Canton’s geographical location in a dialectal transition zone, increased personal mobility, and the growing presence of broadcast media in dialect.