Do Not Place a Pedestrian before a Driver on Shabbat

As my family and I were walking in Israel a question came up concerning driving on shabbas. Is driving one melacha that a driver would be violating or every time the driver presses on the gas or break is its own separate melacha that is being violated?If it is more than one melacha-meaning each time the driver has to stop and start again they are doing a violaton- then in a place like Israel where many of the people who may be driving on shabbas are Jewish (maybe not observant), is it a problem for observant Jews to be crossing the street against the light if it causes them to have to stop or slow down for us, causing them to violate the melacha another time?-Rebecca Schenker

It's important first to articulate the assumption behind your question - that it is wrong to cause another Jew to sin. This is in fact true, and it is based on Chazal's interpretation of "lifnei iver lo titen michshol [do not place a stumbling block before the blind]" (Vayikra 19:14), which is understood to forbid (among other things) causing someone else to commit a sin (the classic example is: giving wine to a nazir). So if starting and stopping a car is indeed melacha, it would be prohibited to cause another Jew to engage in it.

According to Rabbi Dovid Ribiat in his sefer The 39 Melochos (pages 1216-1218), one violates the melacha of mavier (lighting a fire) thousands of times a minute when one drives, because one cylinder of the engine turns approximately 1000 times per minute when the car is idling (=motor running but the car not moving), and each turn is caused by a separate spark. Engines can have 4, 6 or 8 cylinders. When the car drives at a higher speed the engine turns faster, thus increasing the number of sparks released. (Rabbi Ribiat has a fascinating - to this automotive ignoramus, anyway - diagram explaining how a car engine works.) So causing someone to slow down and speed up changes the number of sparks released and would indeed be causing melacha to be done.

And as I'm thinking about your question, it may not be a problem only in Israel. The New York area in general is populated with many Jews, most of whom are not observant, and perhaps even when we cross streets in our own neighborhoods, we should try to avoid forcing cars to stop for us.