Each day in the United States, approximately 9 people are killed and more than 1,000 injured in crashes that are reported to involve a distracted driver. 1 Distracted driving is driving while doing another activity that takes your attention away from driving.
This is called the post-void residual. The bladder has a approximately 500ml capacity. You should feel a desire to void at ~250ml. Urinary retention is measured by using a special type of ultrasound (bladder scan), or by inserting a bladder catheter and measuring how much urine comes out. I'm a 4th year US med student. On one of my urology rotation I asked the Sr. resident this exact question.
A post-void residual urine greater than 50 ml is a significant amount of urine and increases the potential for recurring urinary tract infections. In adults older than 60 years, 50-100 ml of residual urine may remain after each voiding because of the decreased contractility of the detrusor muscle. In chronic retention, ultrasound of the bladder may show massive increase in bladder capacity (normal capacity is 400-600 ml). Determining the serum prostate-specific antigen (PSA) may help diagnose or rule out prostate cancer, though this is also raised in BPH and prostatitis.