Yellow Cab Co. in Chicago is offering to give ads a ride. So far no one has signed up, but the company is pitching 4-by-5-inch ads, displayed in acrylic frames, angled to eye level and illuminated on the cab's interior roof. The minimum commitment is $1,600 for an ad to appear in 100 cabs for three months. The rate goes down if the ad is placed for a longer period or is placed in more cabs. Currently, the space is being used for public service announcements.