Compare trip times by transport mode.
Comparing modes fairly means comparing door to door, and that changes the answer on short urban trips more than people expect. Driving's average speed including congestion, circling for a space and walking from it is often 15 to 20 mph in a city, not the 30 mph limit. Transit adds walking and waiting to its in-vehicle time, which is why a 22-minute ride is really a 36-minute journey. Cycling has almost no access overhead and a steady speed that congestion does not touch, which is why it frequently wins outright under about five miles. The crossover distance where driving takes over is usually further out than intuition suggests.
Trip Time Comparison
Driving minutes = miles x 60 / average speed; transit minutes = in-vehicle + access and wait; cycling minutes = miles x 60 / cycling speed; fastest = min of the three
Commonly three to six miles. Cycling has no parking or waiting overhead and a speed congestion does not affect, so it wins whenever driving's average door-to-door speed falls below about 15 mph.
Time a few real journeys from leaving the house to arriving on foot at the destination, and divide by distance. Most people are surprised how low the resulting figure is.