The uniform arrival and uniform service rates observed on an approach road to a signalized intersection are $20$ and $50$ vehicles/minute, respectively. For this signal, the red time is $30 \: s$, the effective green time is $30 \: s$, and the cycle length is $60 \: s$. Assuming that initially there are no vehicles in the queue, the average delay per vehicle using the approach road during a cycle length (in $s$, round off to $2$ decimal places) is __________